Maternal smoking and congenital malformations

Abstract: About two percent of Swedish newborn infants are born with a significant congenital malformations which is reported to the Swedish Registry of Congenital Malformations (RCM), and/or the Swedish Medical Birth Registry (MBR). In spite of persistent public health recommendations, about 16% of the Swedish pregnant women are smoking during pregnancy (1996). It is well known that maternal smoking during pregnancy confers an increased risk for intrauterine growth retardation, preterm birth, perinatal death, and sudden infant death syndrome. If maternal smoking is also associated with congenital malformations, this would be of great public interest. Congenital malformations are heterogenic from an etiological point of view. When investigating the putative effect of maternal smoking it is thus essential to split the group of malformations into adequate subgroups. This requires a huge study population (because a specific congenital malformations is a rare event) as well as detailed diagnoses of the cases. There are not many registries which could satisfy both demands, however, and the literature regarding maternal smoking and congenital malformations is somewhat equivocal. Since 1983, prospectively collected informations on smoking during early pregnancy is reported to the MBR and by now the Swedish health registries provide a unique opportunity to gather information on 1.4 million births for which maternal smoking habits during pregnancy are known. After a thorough investigation and adjustment of various confounders (such as year of birth, maternal age, parity, involuntary childlessness, maternal educational level, alcohol and recreational drug use etc.) the main findings of the present study was that the overall association between maternal smoking and congenital malformations is negligible. A strong heterogeneity of the risk estimates suggests the existence of an increased or decreased risk (causal or not) for some malformations in infants of smoking mothers. The significant findings were 1) a positive association between maternal smoking and orofacial clefts, limb reduction defects, kidney malformations, truncus malformations, persistent ductus arteriosus at term, craniosynostosis, and multiple malformations (non-specific), and 2) a significant negative association between maternal smoking and neural tube defects (suggested to be due to early losses of affected conceptions of smoking mothers) and hypospadias (a hypothetical consequence of different smoking habits due to different reproduction histories in case and control mothers). If the positive association between maternal smoking and the above mentioned malformations is causal, about 5% (in Sweden some 23 infants each year) of all those cases are damaged due to maternal smoking during pregnancy, a fact which should be of concern for the society. Even if the risk increase for each individual smoking pregnant woman is negligible, the putative teratogenic effect of maternal smoking may be yet another reason to encourage women to quit smoking when planning a pregnancy.