This study adds to the increasing body of evidence which suggests that obesity, measured by BMI, predisposes women to complicated pregnancies and increased obstetric interventions. We found a linear relationship between increasing body mass index and the risk of developing pre-eclampsia, gestational hypertension, induction of labour and emergency caesarean section. Conversely, low BMI had a protective effect on some obstetric complications.
Previous research has found a strong association between increasing BMI and pregnancy induced hypertension. A meta-analysis of the risk of pre-eclampsia associated with maternal BMI [19] showed that the risk of pre-eclampsia doubled with each 5 to 7 Kg/m2 increase in prepregnancy BMI. We found a 3 times higher risk of pre-eclampsia in obese (BMI 30 to 39.9 Kg/m2) and a 7 times higher risk in morbidly obese (BMI > 40 Kg/m2) primigravid women. We also found a significantly lower risk of pre-eclampsia in underweight women {OR 0.6 (95% CI 0.5 – 0.7)}, a finding corroborated by Sebire et al [16].
Our results agree with earlier reports which have shown an association between increasing BMI and interventions like induced labour [10, 13, 20] and caesarean delivery [8, 10, 14, 23]. Some previous work has also demonstrated a strong link between postpartum blood loss and BMI. Although we found a linear increase in mean postpartum blood loss with increasing BMI, the risk of postpartum haemorrhage, defined as blood loss of more than 500 ml for vaginal delivery and 1000 ml for caesarean delivery, was significantly higher only in the obese category. Other studies have reported conflicting results. While Sebire et al [12] observed a 70% increase in postpartum haemorrhage, Bianco et al [22] found no such difference in the incidence. As measurement of blood loss is subjective, and the definition of postpartum haemorrhage variable, it is difficult to make comparisons across studies. Intuitively, it appears that women with higher body mass index should bleed more, but this is at least in part due to the increased incidence of induced labour and operative deliveries in these women.
In contrast to the majority of studies in the literature [21, 16] our adjusted data failed to show any differences in the risk of preterm delivery (delivery before 37 completed weeks) in the different BMI categories. Cnattingius [11] found no association between preterm delivery before 37 weeks and prepregnancy weight, although the risk of very preterm delivery before 33 weeks was increased in overweight nulliparous women. This was corroborated by our results, which showed that the risk of preterm delivery before 33 weeks was higher in the obese group, but not in the morbidly obese. On the other hand, Sebire et al [12] found that delivery before 32 weeks was significantly less likely in the obese.
With regard to intrauterine growth retardation measured by the adjusted birth weight, we found a strong association with maternal BMI. While the risk of low birth weight (birth weight less than 2,500 g) was higher in underweight women, macrosomia was much more common in the obese and morbidly obese groups. Several studies investigating the relationship of maternal obesity with fetal growth have shown that obese women have an 18 – 26% increased chance of delivering large for date infants, even after controlling for maternal diabetes [12, 22–24].
Yu et al [25] suggest that the rapid fetal growth induced by maternal hyperinsulinaemia coupled with placental insufficiency may result in the antepartum demise of the fetus in obese pregnant women. Indeed this hypothesis has been corroborated by several epidemiological studies [9, 10]. This study found an increased risk of stillbirth in obese, but not morbidly obese women. In reality, there were too few women in the morbidly obese group to comment on this group's association with a rare outcome like stillbirth.
Apart from the slightly increased risk of having a baby with low birthweight, the mothers with BMI < 20 appeared to be at a lower risk of developing any other pregnancy or labour complications compared even to women with BMI in the normal range – a finding corroborated by Sebire et al [16]. Several complications like postpartum haemorrhage, preterm delivery and macrosomia were found to increase linearly with rising BMI, but no longer remained statistically significant in the morbidly obese women after adjusting for confounders. This can be partially explained by the smaller sample size of morbidly obese women and partially by the overwhelming effect of pre-eclampsia, gestational hypertension and interventions during labour and delivery in these women.
The growing interest in obesity in pregnancy has prompted at least two good quality reviews [25, 26] and several primary studies. Most studies have used a retrospective cohort design using data from routinely collected hospital databases [12, 21] or trial data (Weiss – FASTER trial) [24]. In neither case do the data reflect population trends. As the AMND records and stores information on all pregnancy events in a geographically defined area, our data set is truly population based. We have restricted our data between 1976 and 2005, as the principal aim of this study was to examine pregnancy outcomes while minimising the effects of changes in clinical practice over time. Nevertheless, even this limited dataset, shows a rising incidence of obese women booking for antenatal care. Despite restricting this study to nulliparous women delivering singleton babies we achieved a sample size of 24,241 women, which to our knowledge makes it one of the largest studies of this kind. In contast with most retrospective studies, all pregnancy events were concurrently recorded by AMND staff thereby limiting recall bias. The height and weight recorded at the booking antenatal visit were coded using stringent criteria and standard definitions and are subject to validity and consistency checks, making the data completely reliable. More information about coding and quality of the dataset is available at the AMND website [27].
This study, like any other observational study of its kind suffers from several limitations. Firstly, the ideal time to record the baseline height and weight of a pregnant woman is before she has started gaining weight due to gestation. As this is seldom available on a routine database, most researchers have relied on the woman's recall of her pre-pregnancy height and weight, the reliability and standardisation of which is very doubtful [28]. In our study we have relied on height and weight recorded in early pregnancy, before any real impact of gestational weight gain. Still, values recorded in early pregnancy remain an approximation of the pre-pregnancy weight, and therefore subject to bias. Also, exclusion of all women who booked after 16 weeks of gestation could have resulted in selection bias, overweight or underweight women being systematically excluded from the dataset. However, we found an even distribution of the week of antenatal booking visit amongst the different BMI categories, thereby minimising selection bias. Our study used data collected over 30 years, during which time there have been several changes in obstetric protocols, especially with regard to induction of labour and caesarean deliveries and this may have influenced some of the outcomes studied. To account for this, we included year of delivery in the logistic regression model when deriving the adjusted Odds Ratios.
Recent reviews [29] on obesity and pregnancy have highlighted several issues relevant to research and management policy. Firstly, the lack of standard definitions of overweight and obesity makes comparison of findings across studies difficult. While most reports define obesity as an increased body mass index of greater than or equal to 30 Kg/m2 (IOM), others have defined it as increased waist circumference, increased waist – hip ratio or body weight of more than 90 Kg. This makes comparison of studies difficult and may have implications in the management of normal pregnancy, as in the United States, recommended gestational weight gain is dependent on women's prepregnancy BMI categories [30]. Moreover, in most clinics, pre-pregnancy BMI is not recorded routinely, thereby making extrapolation of booking weight or women's recall of prepregnancy weight unreliable. Krishnamoorthy et al [29] suggest that all pregnancies in obese women be acknowledged as high risk and managed according to strict guidelines. Management should include prepregnancy counselling to reduce weight; shared antenatal care and appropriate management of complications. The evidence for obesity as an important complication in pregnancy is mounting; it is time to inform practice based on this evidence.
