Objective Hypertensive diseases in pregnancy as well as smoking are associated with prematurity and intrauterine growth restriction (IUGR). However recent studies have suggested that smoking has a protective effect, reducing the incidence of preeclampsia and gestational hypertension (PE/GH). We set out to investigate the relative effects that PE/GH and smoking have on prematurity and IUGR.

Method Because of ethnic variation associated with smoking, this analysis focused on all British-European women only, from a 2009/2010 routinely collected NHS maternity database (n=27 645). Prematurity was defined as birth <37 weeks gestation, and IUGR as birthweight below the 10th customised centile, adjusted for maternal height, weight, parity as well as fetal sex and gestational age at birth.

Results PE, 1.2% and GH, 4.5% were combined to give sufficient numbers for subgroup analysis. Mothers who smoked throughout pregnancy had significantly lower rates of PE/GH 4.0 versus 6.0%, (OR 0.6, CI 0.4 to 0.9). However, despite lower rates of hypertensive diseases in pregnancy, women who smoked had significantly increased rates of prematurity and IUGR than non-smokers, and these differences were dose dependent. The data are summarised in table 1.

Conclusion Smoking during pregnancy reduces the incidence of hypertensive diseases, or at least their clinical manifestation, regardless of the number of cigarettes smoked. Despite this effect, pregnancies with maternal smoking have a significantly increased risk of prematurity and IUGR.