Preconceptional factors associated with very low birthweight delivery in East and West Berlin: a case control study

  • Ingrid Grimmer1,

    Affiliated with

    • Christoph Bührer1Email author,

      Affiliated with

      • Joachim W Dudenhausen2,

        Affiliated with

        • Andrea Stroux3,

          Affiliated with

          • Horst Reiher4,

            Affiliated with

            • Horst Halle5 and

              Affiliated with

              • Michael Obladen1

                Affiliated with

                BMC Public Health20022:10

                DOI: 10.1186/1471-2458-2-10

                Received: 8 April 2002

                Accepted: 24 June 2002

                Published: 24 June 2002

                Abstract

                Background

                Very low birthweight, i.e. a birthweight < 1500 g, is among the strongest determinants of infant mortality and childhood morbidity. To develop primary prevention approaches to VLBW birth and its sequelae, information is needed on the causes of preterm birth, their personal and social antecedents, and on conditions associated with very low birthweight. Despite the growing body of evidence linking sociodemographic variables with preterm delivery, little is known as to how this may be extrapolated to the risk of very low birthweight.

                Methods

                In 1992, two years after the German unification, we started to recruit two cohorts of very low birthweight infants and controls in East and West Berlin for a long-term neurodevelopmental study. The present analysis was undertaken to compare potential preconceptional risk factors for very low birthweight delivery in a case-control design including 166 mothers (82 East vs. 84 West Berlin) with very low birthweight delivery and 341 control mothers (166 East vs. 175 West).

                Results

                Multivariate logistic regression analysis was used to assess the effects of various dichotomous parental covariates and their interaction with living in East or West Berlin. After backward variable selection, short maternal school education, maternal unemployment, single-room apartment, smoking, previous preterm delivery, and fetal loss emerged as significant main effect variables, together with living in West Berlin as positive effect modificator for single-mother status.

                Conclusion

                Very low birthweight has been differentially associated with obstetrical history and indicators of maternal socioeconomic status in East and West Berlin. The ranking of these risk factors is under the influence of the political framework.

                Background

                The majority of preterm infants, i.e. those born prior to 37 weeks of gestation, do well, thanks to the advances of modern obstetric and neonatal care. However, chances for intact survival of preterm infants decrease with advancing immaturity [1], and very low birthweight (VLBW) infants, i.e. those with a birthweight < 1500 g, continue to constitute an enormous burden on the tangible and intangible resources of families, health care providers, and educational systems. VLBW infants are more likely to undergo recurrent hospitalizations and require special educational assistance, and VLBW sequelae such as motor deficits and cognitive disturbances may persist for life time [2].

                Attempts at averting very preterm birth altogether (primary prevention), intervening prior to impending preterm birth (secondary prevention), and sophisticated treatment of preterm infants (tertiary prevention) to minimize sequelae have met with mixed success. Progress has largely been confined to the latter two of these strategies while primary prevention measures such as social support programs [3] and home uterine monitoring for the detection and treatment of early contractions have so far not yet proven successful [4]. While our knowledge of the natural history of this complex and probably multi-determined health state is very limited [5, 6], the information availabe suggests that risk factors for preterm delivery may predate pregnancy.

                Preterm birth is a world-wide problem, but rates of VLBW differ more than twofold between industrialized countries. Some countries with advanced neonatal care, including the United States, show poor performance in the international ranking of infant mortality rates due to a high proportion of VLBW deliveries [7]. The rate of preterm birth in the United States is double or more that of many other industrialized nations, which has been attributed in part to the increased likelihood of preterm delivery in black women, as compared with white women in the United States. Black/white ratios of infant mortality (2.4) and VLBW (2.6) are indeed very similar [7].

                Some of the discrepancy between the races has been associated with maternal socioeconomic status and subsequent access to preventive health care [8, 9]. Racial disparity in socioeconomic status may be greater than the ability to adjust for it in epidemiological studies [10]. However, the racial difference in the risk of preterm delivery has also been observed in United States Army personnel who have no-cost access to high-quality health care [11]. In the United States, immigrant black women have substantially lower preterm birth rates than black women born in the United States [12, 13]. In the Netherlands where there is easy access to medical care for all inhabitants, adjusted birthweights of Dutch, Turkish, and Carribean were not found to differ significantly. In contrast, very low social class was associated with a slightly reduced birth weight [14].

                While the association between sociodemographic variables and preterm delivery becomes growingly evident, little is known about how these factors may promote the risk of VLBW [15]. Variables predictive of preterm delivery may not be associated with VLBW, while others such as maternal smoking have been found to be associated with delivery before 32 weeks but not with preterm birth itself [16].

                The investigation presented here has seized the historical opportunity provided by the process of German unification since 1989. In both parts of Berlin, geography, nutritional traditions, the degree of urbanization and industrialization, language, and genetic background were virtually identical. Both countries took pride in no-cost access to prenatal care and advanced perinatal medicine available to everybody. However, while medicine had the same scientific roots and similar traditions in both parts of Germany, social and economic systems started to differ vastly after 1945, with an ever-increasing distance after the two German states were founded in 1949 and the wall was built in 1961. Consecutively, the socio-political infrastructure and with that the organization of health care diverged progressively. After the political unification, the percentage of VLBW infants increased and the mortality of VLBW infants decreased in both parts of Germany [17], whereas the pattern of breastfeeding remained different as long as eight years afterwards [18].

                In 1992, two years after the unification, we started to recruit two cohorts of VLBW infants and controls in East and West Berlin for a long-term neurodevelopmental study [19]. The present case-control analysis was undertaken to detect and to compare preconceptional, exclusively parental risk factors associated with VLBW in East and West Berlin.

                Methods

                The original study population consisted of all consecutive VLBW infants (birthweight from 500 to less than 1500 g) born alive (presence of heart beat or pulsating umbilical cord or breathing efforts) between June 15, 1992 and June 15, 1994, in a West Berlin university hospital and a major municipal East Berlin hospital, and all consecutive VLBW infants between April 1, 1993 and June 15, 1994, in the East Berlin university hospital. The two infants born next to a VLBW infant in the same hospital and who displayed a birthweight of at least 2500 g served as controls.

                During the three years the study was conducted, VLBW infants accounted for 0.821 % (199/24252) and 0.895% (608/67882) of all infants born in East and West Berlin, respectively (p = 0.281). Similarly, the ratio of infants with birthweights <1500 g and ≥ 2500 g (199/21492 = 9.25 × 10-3 vs 608/59360 = 10.24 × 10-3) did not differ between East and West Berlin (p= 0.219).

                After informed consent, mothers were asked to participate in the study. Medical history and socioeconomic data were collected via structured questionnaires within the first three days after delivery. Interviews with non-German mothers were conducted with the assistance of a translator. Pregravid body measurements were retrieved from maternity booklets. Education was classified as short (less than 10 years), intermediate (a minimum of 10 years, as required for most skilled job training opportunities), and advanced (college/university entry grade, i.e. a minimum of 12 years).

                Comparisons between groups were performed by χ2 test or Fisher's exact test. Potential associations between effect variables and VLBW delivery were explored by univariate logistic regression analyses. In general, depending on the proportions in the control group, odds ratios between 2.8 (proportion = 5%) and 1.8 (proportion = 30%) can be detected with the given sample size of 166 in the VLBW mothers group and 341 in the control mothers group, a two-sided significance level of 0.05, and a power of 80%. However, the analyses were not determined by one single primary hypothesis but by a bundle of potential associations. Thus, the analysis was exploratory, and P-values have no recognized interpretation under these conditions.

                The relative risk of VLBW delivery was estimated by odds ratios and 95% confidence intervals. Univariate logistic regression analyses were performed separately for East and West Berlin mothers. To control for possible confounding and identify interaction effects, a multivariate logistic regression model [20] was developed including those factors significant (p < 0.05) in the univariate analyses in any of the two groups – with the exception of paternal education and employment status which were positively associated with the corresponding maternal factors to a considerable amount, i.e. Spearman's R = 0.498 and 0.310, respectively. Effect modification through living in East/West Berlin was tested by including location as a factor into the initial multivariate model, together with terms for interaction between covariates and location. The resulting model was obtained by performing backward binary logistic regression analysis. For the variable selection process, P-values < 0.05 for inclusion, and ≥ 0.1 (significance of change when variable removed) for exclusion, respectively, were used. Location, as a design variable of the study, was included into the entire selection process.

                Additionally, we performed the analogous model building procedure with less stringent selection criteria, taking P-values of 0.20 and 0.25 as inclusion and exclusion criteria, respectively.

                Statistical analysis was performed using SPSSWIN10.0 (SPSS Inc, Chicago, Illinois).

                Results

                Study population

                During the study period, a total of 194 VLBW were born to 179 mothers in the particpiating hospitals. Thirteen out of 179 VLBW infant mothers (7.3%) refused to participate, the most frequent reason cited being death of the infant (n = 7). Similarly, 37 out of 378 control infant mothers (9.8%) did so, the largest fraction of them (n = 16) without known reason. Thus, the number of VLBW and control infant mothers actually participating in the study were 166 (82 in East Berlin and 84 in West Berlin) mothers of 190 VLBW infants, and 341 (175 in West Berlin and 166 in East Berlin) mothers of 307 control infants. Refusal rates did not differ significantly between VLBW and control infant mothers (p = 0.427) or between East and West Berlin mothers (28 of 276 [10.1%] vs. 22 of 281 [7.8%], p = 0.375).

                Prevalence of potential risk factors in East and West Berlin

                In Table 1, the prevalences of preconceptional factors potentially related to VLBW delivery in the study population are presented. In general, most of these factors occured more often in the group of VLBW infant mothers than in the control group. With the exception of a higher rate of pregnancies after assisted reproduction in the West as compared to the East Berlin group, obstetric history data did not differ considerably. The percentages of single mothers, mothers living in single-room apartments, and very slim or very young mothers were higher in the East Berlin group. Also, in East Berlin there were more mothers who were unemployed. In contrast, the rates of paternal unemployment did not differ considerably. Elder women (> 35 years of age) and women who were migrants from developing countries (mostly Turkey in West Berlin, and Vietnam in East Berlin) constituted a larger proportion of the West than of the East Berlin group. The levels of both maternal and paternal education were also distributed differently in the East and West Berlin groups, with larger fractions having either low (less than 10 years) or high (12 years or more) school education in West Berlin, as opposed to East Berlin.
                Table 1

                Prevalence of potential preconceptional risk factors in East and West Berlin*

                Potential risk factors

                Control group

                VLBW

                 

                East Berlin n = 166

                West Berlin n = 175

                East Berlin n = 82

                West Berlin n = 84

                Obstetric history

                    

                   previous preterm delivery

                4.8

                8.0

                15.9

                14.3

                   previous fetal loss

                36.1

                36.0

                50.0

                53.6

                   previous induced abortion

                21.7

                20.6

                29.3

                31.0

                   assisted reproduction

                1.8

                5.7

                2.5

                8.3

                   prima para

                49.4

                51.4

                51.2

                56.0

                Other maternal variables

                    

                   low net income (< 800 DM/m)

                1.2

                2.3

                7.3

                7.1

                   single mother

                16.3

                4.6

                31.7

                20.2

                   school education

                    

                less than 10 years

                15.1

                25.7

                34.1

                45.2

                10 to 11 years

                57.8

                37.1

                51.2

                28.6

                at least 12 years

                27.1

                37.1

                13.4

                26.2

                   single-room apartment

                9.0

                3.4

                18.3

                10.7

                   unemployed during pregnancy

                19.9

                6.9

                35.4

                16.7

                   migrant from developing country

                7.2

                18.3

                13.4

                26.2

                   age less than 20 years

                5.4

                1.7

                8.5

                4.8

                   age more than 35 years

                6.6

                14.9

                11.0

                19.0

                   body mass index less than 20

                34.5

                22.5

                31.6

                20.0

                   body mass index more than 30

                5.5

                4.7

                3.8

                3.8

                   smoking during pregnancy

                26.5

                26.9

                40.7

                44.0

                Paternal variables

                    

                   unemployment

                12.7

                9.7

                24.7

                15.5

                   school education

                    

                less than 10 years

                15.3

                28.9

                30.4

                40.7

                10 to 11 years

                53.4

                26.0

                48.1

                25.9

                at least 12 years

                31.3

                45.1

                21.5

                33.3

                * Values are (valid) percentages of total control and VLBW mothers population in East and West Berlin, respectively.

                Factors associated with VLBW infant delivery in East and West Berlin

                Separate univariate logistic models for East and West Berlin data yielded partly differing results. Among the obstetric history variables, only previous preterm delivery in East Berlin, and previous fetal loss (stillbirth or spontaneous abortion) in both sites were significantly related to VLBW infant delivery (Table 2).
                Table 2

                Odds Ratios (95% confidence intervals) for factors related to VLBW in East and West Berlin*

                potential risk factors for VLBW

                 

                East Berlin

                West Berlin

                  

                VLBW

                  

                VLBW

                  
                  

                no

                yes

                OR (95% CI)

                p

                no

                yes

                OR (95% CI)

                p

                  

                166

                82

                  

                175

                84

                  

                previous preterm delivery

                no

                157

                69

                3.70 (1.47–9.33)

                0.006

                161

                72

                1.92 (0.85–4.35)

                0.120

                 

                yes

                8

                13

                  

                14

                12

                  

                previous fetal loss

                no

                106

                41

                1.77 (1.03–3.02)

                0.038

                112

                39

                2.05 (1.21–3.48)

                0.008

                 

                yes

                60

                41

                  

                63

                45

                  

                previous induced abortion

                no

                130

                58

                1.49 (0.82–2.73)

                0.191

                139

                58

                1.73 (0.96–3.12)

                0.068

                 

                yes

                36

                24

                  

                36

                26

                  

                assisted reproduction

                no

                163

                79

                1.38 (0.23–8.40)

                0.730

                164

                77

                1.49 (0.55–4.07)

                0.435

                 

                yes

                3

                2

                  

                10

                7

                  

                prima para

                no

                82

                40

                1.03 (0.60–1.74)

                0.927

                90

                37

                1.35 (0.80–2.27)

                0.267

                 

                yes

                84

                42

                  

                85

                47

                  

                low net income

                no

                164

                76

                6.47 (1.28–32.8)

                0.024

                171

                78

                3.29 (0.90–11.00)

                0.071

                 

                yes

                2

                6

                  

                4

                6

                  

                single mother status

                no

                139

                56

                2.39 (1.28–4.45)

                0.006

                167

                67

                5.30 (2.18–12.90)

                < 0.001

                 

                yes

                27

                26

                  

                8

                17

                  

                low maternal education**

                no

                141

                53

                3.09 (1.66–5.74)

                < 0.001

                130

                46

                2.39 (1.38–4.13)

                0.002

                 

                yes

                25

                29

                  

                45

                38

                  

                single-room apartment

                no

                151

                67

                2.25 (1.04–4.87)

                0.039

                169

                75

                3.38 (1.16–9.84)

                0.025

                 

                yes

                15

                15

                  

                6

                9

                  

                maternal unemployment

                no

                133

                53

                2.21 (1.22–4.00)

                0.009

                163

                70

                2.72 (1.20–6.17)

                0.017

                 

                yes

                33

                29

                  

                12

                14

                  

                migrant from developing country

                no

                154

                71

                1.99 (0.84–4.72)

                0.119

                143

                62

                1.59 (0.86–2.95)

                0.145

                 

                yes

                12

                11

                  

                32

                22

                  

                maternal age < 20 years

                no

                157

                75

                1.63 (0.58–4.54)

                0.351

                172

                80

                2.87 (0.63–13.10)

                0.175

                 

                yes

                9

                7

                  

                3

                4

                  

                maternal age > 35 years

                no

                155

                73

                1.74 (0.69–4.38)

                0.241

                149

                68

                1.35 (0.68–2.68)

                0.393

                 

                yes

                11

                9

                  

                26

                16

                  

                body mass index < 20

                no

                108

                54

                0.88 (0.50–1.56)

                0.654

                131

                64

                0.86 (0.45–1.66)

                0.657

                 

                yes

                57

                25

                  

                38

                16

                  

                body mass index > 30

                no

                156

                76

                0.68 (0.18–2.60)

                0.577

                161

                77

                0.78 (0.20–3.04)

                0.725

                 

                yes

                9

                3

                  

                8

                3

                  

                smoking during pregnancy

                no

                122

                49

                1.87 (1.07–3.27)

                0.029

                128

                47

                2.14 (1.24–3.70)

                0.006

                 

                yes

                44

                33

                  

                47

                37

                  

                paternal unemployment

                no

                145

                61

                2.26 (1.15–4.48)

                0.019

                158

                71

                1.70 (0.78–3.69)

                0.179

                 

                yes

                21

                20

                  

                17

                13

                  

                low paternal education**

                no

                138

                55

                2.41 (1.27–4.58)

                0.007

                123

                48

                1.69 (0.97–2.94)

                0.062

                 

                yes

                25

                24

                  

                50

                33

                  

                * Analysed using univariate logistic regressions separately for East and West Berlin mothers.** school education less than 10 years

                Most variables associated with low socioeconomic status of the mother, such as low education (less than 10 years of school), living in a single-room apartment, maternal unemployment, or single-mother status, were associated with an increased risk of delivery of a VLBW infant in both groups, as was smoking during pregnancy. Odds ratios usually amounted from about 2 to more than 3 for low maternal education in East Berlin and single-room apartment in West Berlin. A remarkable difference could be found between odds ratios for single-mother status in East Berlin (OR = 2.39) as compared to West Berlin (OR = 5.30) (Table 2). Low net income, low paternal education, and paternal unemployment yielded significant association with VLBW only in the East Berlin group.

                Results of the multivariate logistic model: effect modification through location

                After control for confounding and idenfication of interactions between main effect variables and living in East or West Berlin by multivariate analysis of the total sample, previous preterm delivery, living in a one-room apartment, and low maternal education emerged as the strongest independent explanatory variables (OR ≥ 2), followed by smoking during pregnancy, maternal unemployment, and previous fetal loss (OR > 1.5). Moreover, interaction between location and marital status could be identified: single-mother status turned out to increase the risk of VLBW delivery stronger in West Berlin as compared to East Berlin. (Table 3) The chance of VLBW delivery for single mothers living in West Berlin was nearly four times as high as compared to that of mothers with a partner who were living in East Berlin. In contrast, in East Berlin single mother status showed a relatively weak effect as compared to non-single status there (OR = 1.5). Location, by study design, did not reveal any considerable effect on mothers with companion.
                Table 3

                Factors associated with VLBW infant delivery after adjustment for potential confounding and assessment for effect modification

                main effect variables and interaction terms in the resulting model*

                OR (95% CI)

                previous preterm delivery

                2.31 (1.20–4.44)

                low maternal education

                2.00 (1.31–3.19)

                single-room apartment

                1.95 (0.98–3.87)

                previous fetal loss

                1.64 (1.09–2.46)

                maternal unemployment

                1.60 (0.94–2.72)

                smoking during pregnancy

                1.51 (0.99–2.31)

                no single mother, living in East Berlin**

                1.0

                no single mother, living in West Berlin

                0.96 (0.61–1.51)

                single mother, living in East Berlin

                1.53 (0.78–3.00)

                single mother, living in West Berlin

                3.86 (1.50–9.95)

                * Obtained after backward multiple logistic regression** reference category

                Discussion

                Among infants epidemiologically classified as preterm (born before 37 weeks of gestation) or having low birth weight (< 2500 g), VLBW infants constitute approximately 15%. However, long-term sequelae of preterm birth are virtually confined to VLBW infants, and VLBW rather than preterm birth by itself constitutes a major public health issue.

                Our findings confirm the importance of sociodemographic variables for VLBW delivery by mothers brought up in two societies with vastly differing economic systems but similar standards of medical care, including accessibility. While technologically advanced equipment may still have been used more widely in West Berlin and efforts aimed at regionalization of high-risk pregnancies may have met with more success in East Berlin during the first five-year period following political unification, the associations of most variables with VLBW were similar in East and West Berlin.

                Nevertheless, for single-mother status the univariate analyses revealed a considerable difference in odds ratios between the two parts of Berlin, being in West Berlin more than twice that in East Berlin. Multivariate logistic regression with inclusion of interaction terms verified single-mother status to be a risk factor strongly depending on the part of Berlin the mother lived in, confirming the assumption that certain risk factors act differently in different political frameworks. The important effect of living in East or in West Berlin on single-mother status as a VLBW risk factor partly resides in different attitudes fostered by the state (in East Berlin) or churches (in West Berlin). Single mother status has been largely acceptable in East Berlin, and the percentage of single mothers in East Berlin was twice that in West Berlin. Single mothers had easier access to subsidized housing in East Berlin. In particular, single-mother status was much less indicative of low socio-economic status in East Berlin, as compared to West Berlin. Traditionally, recognized marriage is being advocated by both the protestant and catholic church, and churches do have a strong legal position in West Germany (and hence West Berlin), whereas religion has been viewed as a completely private (and rather detrimental) matter in East Germany.

                In the univariate analyses, the father's social status was significantly related to VLBW in East Berlin while it had little impact on VLBW in the West Berlin group. These findings are in line with recently published observations from Denmark [21] which failed to identify paternal determinants of preterm delivery.

                In the multivariate analysis, no more significant influence of paternal variables could be detected (at the 0.05 level). However, allowing P-values of 0.20 and 0.25 as inclusion and exclusion criteria for variable selection, low net income (OR = 2.1, CI = [0.7, 6.9]), paternal unemployment – as independent factor (OR = 1.6, CI = [0.8, 3.4]) as well as interacting with living in East Berlin (OR [father unemployed and living in East Berlin/father employed and living in West Berlin] = 2.0, CI = [0.9, 4.2]) – and maternal unemployment with increased effect when living in West Berlin, were obtained as additional factors associated with VLBW infant delivery. This supports the hypothesis that, under different socio-political circumstances, the influence of socio-demographic factors on VLBW delivery partly differs substantially.

                As the present study is strictly based on birthweight to avoid the inaccuracies and uncertainties inherent to gestational age calculations, it cannot discriminate between determinants of intrauterine growth impairment and early onset of labor. Several variables related to increased VLBW risk in this study have been reported previously also to be associated with preterm birth. These include indicators of low social status such as low school education, single-mother status, low income [16, 1, 2225] previous preterm delivery or fetal loss [26, 27] and smoking [2830]. These factors appear to be related to both preterm (before 37 weeks of gestation) and very preterm (before 32 weeks of gestation) delivery [15]. In contrast, medical variables previously reported to be associated with preterm birth or low birthweight, such as low maternal body weight, teenage pregnancy or old age of the mother [23, 3133] were not associated with VLBW. This supports the notion that despite the inadvertent overlap between VLBW and preterm birth, VLBW should be dealt with separately [34, 35].

                Conclusions

                The advances in the management of impending preterm delivery and in the intensive care of preterm infants are in sharp contrast to the failure to reduce the rate of VLBW during the last 25 years. While we do not know how risk factors present before pregnancy translate into biologic events that trigger labor prematurely and limit intrauterine growth, our findings confirm the assumption that there are strong associations between low parental, especially maternal, socioeconomic status, cigarette smoking, and VLBW. Moreover, VLBW was differently associated with marital status in East and West Berlin.

                As physicians treat individual patients but not societies, VLBW may be a public health issue awaiting political treatment; still approaches to compensate for social inequalities regarding access and utilization of preventive health care resources should be specificly promoted.

                Declarations

                Acknowledgements

                We would like to thank Catharina Luhr, Claudia Homm, and Gerhard Wellmitz for logistical support, and Boris Metze for help with data management. Supported by a grant from the Federal Department of Research and Technology (BMFT) on Public Health (07PHBF01), Project B3.

                Authors’ Affiliations

                (1)
                Department of Neonatology, Charité Virchow Hospital; Humboldt University
                (2)
                Department of Obstetrics, Charité Virchow Hospital; Humboldt University
                (3)
                Berlin Center Public Health; Department of Medical Informatics, Biometry and Epidemiology, University Hospital Benjamin Franklin, Free University
                (4)
                Department of Obstetrics and Gynecology, Friedrichshain Municipal Hospital
                (5)
                Department of Obstetrics and Gynecology, Charité Campus Mitte, Humboldt University

                References

                1. Wood NS, Marlow N, Costeloe K, Gibson AT, Wilkinson AR: Neurologic and developmental disability after extremely preterm birth. EPICure Study Group. N Engl J Med 2000, 343: 378–384.View ArticlePubMed
                2. Hack M, Flannery DJ, Schluchter M, Cartar L, Borawski E, Klein N: Outcomes in young adulthood for very-low-birth-weight infants. N Engl J Med 2002, 346: 149–157.View ArticlePubMed
                3. Villar J, Farnot U, Barros F, Victora C, Langer A, Belizan JM: A randomized trial of psychosocial support during high-risk pregnancies. The Latin American Network for Perinatal and Reproductive Research. N Engl J Med 1992, 327: 1266–1271.View ArticlePubMed
                4. Grimes DA, Schulz KF: Randomized controlled trials of home uterine activity monitoring: a review and critique. Obstet Gynecol 1992, 79: 137–142.PubMed
                5. Lumley J: The epidemiology of preterm birth. Baillieres Clin Obstet Gynaecol 1993, 7: 477–498.View ArticlePubMed
                6. Berkowitz GS, Papiernik É: Epidemiology of preterm birth. Epidemiol Rev 1993, 15: 414–443.PubMed
                7. Guyer B, Hoyert DL, Martin JA, Ventura SJ, MacDorman MF, Strobino DM: Annual summary of vital statistics – 1998. Pediatrics 1999, 104: 1229–1246.View ArticlePubMed
                8. Lieberman E, Ryan KJ, Monson RR, Schoenbaum SC: Risk factors accounting for racial differences in the rate of premature birth. N Engl J Med 1987, 317: 743–748.PubMed
                9. Kleinman JC, Kessel SS: Racial differences in low birth weight. Trends and risk factors. NEnglJMed 1987, 317: 749–753.
                10. Berg CJ, Wilcox LS, d'Almada PJ: The prevalence of socioeconomic and behavioral characteristics and their impact on very low birth weight in black and white infants in Georgia. Matern Child Health J 2001, 5: 75–84.View ArticlePubMed
                11. Rawlings JS, Weir MR: Race- and rank-specific infant mortality in a US military population. Am J Dis Child 1992, 146: 313–316.PubMed
                12. Singh GK, Yu SM: Adverse pregnancy outcomes: differences between US- and foreign-born women in major US racial and ethnic groups. Am J Public Health 1996, 86: 837–843.View ArticlePubMed
                13. David RJ, Collins JW: Differing birth weight among infants of U.S.-born blacks, African-born blacks, and U.S.-born whites. N Engl J Med 1997, 337: 1209–1214.View ArticlePubMed
                14. Verkerk PH, Zaadstra BM, Reerink JD, Herngreen WP, Verloove Vanhorick SP: Social class, ethnicity and other risk factors for small for gestational age and preterm delivery in The Netherlands. Eur J Obstet Gynecol Reprod Biol 1994, 53: 129–134.View ArticlePubMed
                15. Ancel PY, Saurel Cubizolles MJ, Di Renzo GC, Papiernik É, Breart G: Social differences of very preterm birth in Europe: interaction with obstetric history. Europop Group. Am J Epidemiol 1999, 149: 908–915.PubMed
                16. Peacock JL, Bland JM, Anderson HR: Preterm delivery: effects of socioeconomic factors, psychological stress, smoking, alcohol, and caffeine. BMJ 1995, 311: 531–535.PubMed
                17. Nolte E, Brand A, Koupilová I, McKee M: Neonatal and postneonatal mortality in Germany since unification. J Epidemiol Community Health 2000, 54: 84–90.View ArticlePubMed
                18. Dulon M, Kersting M, Schach S: Duration of breastfeeding and associated factors in Western and Eastern Germany. Acta Pædiatr 2001, 90: 931–935.View ArticlePubMed
                19. Heiser A, Curcin O, Luhr C, Grimmer I, Metze B, Obladen M: Parental and professional agreement in developmental assessment of very-low-birthweight and term infants. Dev Med Child Neurol 2000, 42: 21–24.View ArticlePubMed
                20. Hosmer DW, Lemeshow S: Model-building strategies and methods for logistic regression. In: Applied Logistic Regression. John Wiley & Sons, New York 1989, 82–118.
                21. Basso O, Olsen J, Christensen K: Low birthweight and prematurity in relation to paternal factors: a study of recurrence. Int J Epidemiol 1999, 28: 695–700.View ArticlePubMed
                22. Leong WP, Viegas OA, Ratnam SS: Premature childbirth: social and behavioural risks in Singapore. J Biosoc Sci 1993, 25: 465–472.View ArticlePubMed
                23. Olsen P, Laara E, Rantakallio P, Jarvelin MR, Sarpola A, Hartikainen AL: Epidemiology of preterm delivery in two birth cohorts with an interval of 20 years. Am J Epidemiol 1995, 142: 1184–1193.PubMed
                24. Nordentoft M, Lou HC, Hansen D, Nim J, Pryds O, Rubin P, Hemmingsen R: Intrauterine growth retardation and premature delivery: the influence of maternal smoking and psychosocial factors. Am J Public Health 1996, 86: 347–354.View ArticlePubMed
                25. Algert C, Roberts C, Adelson P, Frommer M: Low birth-weight in NSW, 1987: a population-based study. Aust N Z J Obstet Gynaecol 1993, 33: 243–248.View ArticlePubMed
                26. Basso O, Olsen J, Christensen K: Risk of preterm delivery, low birthweight and growth retardation following spontaneous abortion: a registry-based study in Denmark. Int J Epidemiol 1998, 27: 642–646.View ArticlePubMed
                27. Malloy MH: Risk of previous very low birth weight and very preterm infants among women delivering a very low birth weight and very preterm infant. J Perinatol 1999, 19: 97–102.View ArticlePubMed
                28. Meis PJ, Michielutte R, Peters TJ, Wells HB, Sands RE, Coles EC, Johns KA: Factors associated with preterm birth in Cardiff, Wales. I. Univariable and multivariable analysis. Am J Obstet Gynecol 1995, 173: 590–596.View ArticlePubMed
                29. Orr ST, James SA, Miller CA, Barakat B, Daikoku N, Pupkin M, Engstrom K, Huggins G: Psychosocial stressors and low birthweight in an urban population. Am J Prev Med 1996, 12: 459–466.PubMed
                30. Cnattingius S, Granath F, Petersson G, Harlow BL: The influence of gestational age and smoking habits on the risk of subsequent preterm deliveries. N Engl J Med 1999, 341: 943–948.View ArticlePubMed
                31. Rodriguez C, Regidor E, Gutierrez Fisac JL: Low birth weight in Spain associated with sociodemographic factors. J Epidemiol Community Health 1995, 49: 38–42.View ArticlePubMed
                32. Wildschut HI, Nas T, Golding J: Are sociodemographic factors predictive of preterm birth? A reappraisal of the 1958 British Perinatal Mortality Survey. Br J Obstet Gynaecol 1997, 104: 57–63.PubMed
                33. Goldenberg RL, Iams JD, Mercer BM, Meis PJ, Moawad AH, Copper RL, Das A, Thom E, Johnson F, McNellis D, Miodovnik M, van Dorsten JP, Caritis SN, Thurnau GR, Bottoms SF: The preterm prediction study: the value of new vs standard risk factors in predicting early and all spontaneous preterm births. NICHD MFMU Network. Am J Public Health 1998, 88: 233–238.View ArticlePubMed
                34. Ancel PY, Saurel-Cubizolles MJ, Di Renzo GC, Papiernik É, Breart G: Very and moderate preterm births: are the risk factors different? Br J Obstret Gynaecol 1999, 106: 1162–1170.
                35. Rauh VA, Andrews HF, Garfinkel RS: The contribution of maternal age to racial disparities in birthweight: a multilevel perspective. Am J Public Health 2001, 91: 1815–1824.View ArticlePubMed
                36. Pre-publication history

                  1. The pre-publication history for this paper can be accessed here:http://​www.​biomedcentral.​com/​1471-2458/​2/​10/​prepub

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                © Grimmer et al 2002

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