The aim of this study was to examine pregnancy outcomes among smoking and non-smoking pregnant women aged less than 35 years and 35 years or older.
There were fewer women who were smoking during pregnancy among women aged 35 years or older that in the group of women under 35, but the former groups’ behavioural risk profile and outcome results clearly demonstrated that older smoking women are a distinct high risk group.
Smoking increased the risks of preterm birth, SGA, LBW, and fetal death in all pregnant women, but even more so in older women.
Maternal age of 35 years or more independently of smoking appeared to increase the risks of preterm delivery, fetal death, preeclampsia and LBW. However, the combination of maternal age of 35 years or more and smoking was significantly increasing the risks of these outcomes in the subgroup analyses performed, showing that the two independent risks, smoking and advanced maternal age, are additive. This reflected two important health issues. First, advanced maternal age alone increased the risk of adverse pregnancy outcome. Second, the high risk group identified by maternal smoking was smaller but even less health conscious in the case of older than younger women. In other words, smoking is a more powerful marker of risky health behaviour among older than younger women.
The incremental risk caused by smoking was clearly higher in older than in younger women in all studied outcomes, even though the effect on fetal growth was clinically the most important.
Our findings are in line with previous studies reporting adverse pregnancy outcomes in smoking women [4, 6–8, 14] and especially with the one reporting that the smoking-related risk of SGA increases with maternal age [7, 14]. Smoking during pregnancy is an important health problem associated with adverse outcomes, particularly fetal growth restriction and preterm birth, which can both have far-reaching health consequences into adult life [15].
The toxic effects of smoking may influence fetal growth more among older smoking women and the longer exposure in the mother to the harmful effects of smoking may also be more damaging to the fetus [7, 14].
In the present study, older women who were smoking were more often overweight than their non-smoking counterparts, but they had had less infertility treatments than non-smokers. On the other hand, smoking and obesity are known to impair fertility. The discrepancy between the known increasing rate of fertility problems and the observed underuse of fertility treatments probably reflected the fact that smokers were less health conscious and sought health care less often than their non-smoking counterparts. In the present study, the study population of the older smoking pregnant women seems to have been positively selected due to their successful pregnancy in spite of the smoking-related risks affecting fertility altogether.
In the present study, smokers were more often unmarried. Married adults are considered to be in better health than unmarried ones and the economic situation of unmarried women is also likely to be worse than of those who are married. Poor health consciousness and smoking are related to being less well-educated and more often unemployed with more alcohol consumption, pregnancy terminations, and untreated infections [16, 17].
It has been reported that women who smoke during pregnancy tend to under-report their smoking [18]. Maternal under-reporting has significant implications on the validity of research in this area, producing underestimates of smoking prevalence and overestimates of the non-smoking/quit smoking status [19]. It is also possible that older pregnant women report smoking more accurately than younger ones [8]. However, the strength of this study is a large population-based register data comprising almost 700 000 births. It has been shown in several studies comparing the internal validity that, in Finnish health registries, the validity and coverage are good as all events are included in the data and the registries comply with reality [20]. The information of health registries provide a highly complete and high-quality source of information that can be utilized, for example, in scientific research [21].
It has been suggested that smoking acts as a marker for other unhealthy habits, exposures, or differences in behaviour or socioeconomic status, but they cannot be measured using birth certificates or registries [8]. However, in Finland, pregnant women are relatively homogenous and they receive similar antenatal and obstetric care, in which case the influence of socioeconomic factors on pregnancy outcome is limited.
The results of the current study suggest and confirm earlier implications that, especially from a public health viewpoint, more attention should be paid to older smoking women during their pregnancy due to an increased risk for impaired fetal growth and preterm delivery as well as other adverse pregnancy outcomes that are harmful to the newborn. In one study, 70% of those who smoked during their first pregnancy continued smoking in a sequential pregnancy, which highlights the importance of smoking cessation during the first pregnancy [22].
