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Few studies have investigated the association hetween hody mass index and fecundability, that is, the ability to conceive in a menstrual cycle, among fertile women with normal menstrual cycle pattern. We examined the independent and combined effects of duration and regularity of the menstrual cycle, body mass index, and fecundability from records on pregnant women attending antenatal care at Odense University Hospital, Denmark, between 1972 and 1987. We included only the first birth of each woman who had planned pregnancies and no pre-pregnancy disease (N = 10,903). We estimated the fecund ability odds ratio (FR) as the odds of conception in a menstrual cycle. After adjusting for confounders, the fecundability for women with a body mass index >25 kg/m2 was lower than for women with a body mass index of 20–25 kg/m2 [FR = 0.77; 95% confidence interval (CI) = 0.70–0.84]. FR was lower for women with long (>35 days) (FR = 0.74; 95% CI = 0.63–0.87) or irregular cycles (FR = 0.78; 95% CI = 0.70–0.87), even when their body mass index was within the normal range (20–25 kg/m2) and/or their cycles were regular.