Obesity has become a health problem in affluent societies, but few studies have investigated its effect on subfertility. Previous studies were based on select groups of women, focused mainly on ovulatory dysfunctions, and yielded controversial results. The authors evaluated the effect of body mass index on delayed conception by using a European population-based survey of pregnant women from five countries. Delayed conception was defined as a time to pregnancy that exceeded 9.5 months of unprotected intercourse. During 1992, 4,035 pregnant women from well-defined geographic areas were recruited consecutively at antenatal clinics or hospitals after at least 20 weeks of gestation. For women smokers, after adjustment for sociodemographic, biologic, and lifestyle-related factors, there was a strong association between obesity (body mass index of > or =30 kg/m2) and delayed conception (odds ratio = 11.54, 95% confidence interval: 3.68, 36.15) and also an increased risk for women whose body mass index was <20 kg/m2 (odds ratio = 1.70; 95% confidence interval: 1.01, 2.83). The same analysis conducted for women nonsmokers showed no association. The authors concluded that for women who achieve a clinically detectable pregnancy, those who are underweight or obese require a longer time to conceive only if they also smoke.