This study aimed to assess the effect of a hospital-based smoking cessation intervention delivered by midwives during routine antenatal and postnatal care on the smoking habits of pregnant women and their partners. At the first antenatal visit, women in the intervention group (n = 110) were given a demonstration of the immediate effects of smoking on foetal heart rate, brief smoking cessation advice and smoking cessation booklets for themselves and their partners; at delivery, they were given brief advice and a booklet about postpartum cessation. Compared with an historical control group who received usual care (n = 110) and assuming those lost to follow-up continued to smoke, biochemically-verified maternal cessation sustained from at least 24 weeks gestation to late pregnancy was 6.4% in the intervention group and 1.8% in the comparison group. However, there was no difference between maternal quit rates at six months postpartum. Partners were more likely to try to quit in the intervention group, but quit rates did not differ. Exposure to the intervention was not associated with increased levels of psychological distress, as measured by the 12-item General Health Questionnaire. We conclude that this type of intervention, when implemented by staff during routine care, is probably associated with a small improvement in maternal cessation in pregnancy, similar to that produced by minimal advice to quit smoking.