Smoking during pregnancy and risk of oral clefts

 

Anatomically, there are two types of oral clefts: cleft lip, usually with cleft palate; and isolated cleft palate. Pathologically, both types are divided into syndromic (babies having other birth defects) and non-syndromic. Cleft lip/palate occurs in approximately 1 in every 1,000 white babies. Isolated cleft palate occurs in approximately 1 in every 2,000 white babies. The causes of oral clefts are not well understood. This meta-analysis aims to estimate the association between cigarette smoking during pregnancy and the risk of having a child with a non-syndromic oral cleft.

Message

Smoking in the first trimester of pregnancy is associated with a slightly higher risk of having a child with an oral cleft.

Reference

DF Wyszynski et al. Maternal cigarette smoking and oral clefts: a meta-analysis. Cleft Palate-Craniofacial Journal 1997 34: 206-210.

Search

The literature was searched using MEDLINE and Current Contents between 1966 and 1996. Bibliographies of retrieved articles were then reviewed. Three experts were also asked for reports that had not been identified with these searches. To be included, cohort and case-control studies had to have (i) examined oral cleft patients (cleft lip/palate, isolated cleft palate, or both) and a comparison group and (ii) recorded cigarette smoking during the first trimester of pregnancy. When one set of results had been analysed in two different studies, the set with the largest sample size was included.

Nine case-control and two cohort studies were identified. All studies examined cleft lip/palate (2,807 cases and 107,024 comparisons). All except two also examined isolated cleft palate (759 cases and 39,649 comparisons). Participant numbers in individual studies varied widely. For example, two studies contributed 91% of comparisons in the cleft lip/palate studies and one study contributed 84% of comparisons in the cleft palate studies. Results of both study designs were pooled.

Results

Cigarette smoking in the first trimester was associated with a small increased risk of having a child with a cleft lip/palate (odds ratio 1.29, 95% confidence interval 1.18 to 1.42) and a child with isolated cleft palate (odds ratio 1.32, 95% confidence interval 1.09 to 1.60).

The authors report that seven studies had adjusted for maternal age, but which ones, and whether any other adjustments were made was not discussed.

Comment

Studies differed in design and participant selection. There was insufficient information on adjustments made to results (e.g. had the studies, contributing the most participants, adjusted for maternal age?) and there was no information on how cigarette smoking was assessed. With this material we can say that smoking is associated with a small increased risk of having a child with an oral cleft, it is just not possible to quantify that risk exactly. But a quick back of envelope calculation indicates that smoking might increase the risk of having a baby with a cleft lip or palate from 1 in 1000 to about 1 in 800.