Pregnancy length was calculated from ultrasonic measurement of the fetal biparietal diameter (BPD) in the second trimester and compared with the corresponding length calculated from the last menstrual period (LMP) in 865 women with exactly 28-day cycles and spontaneous onset of labor. The effects of maternal age, parity, smoking and sex of the child on BPD-based pregnancy length were evaluated in 1998 women with spontaneous onset of labor. The mean pregnancy length was 280.6 days when based on BPD and 283.6 days when based on LMP. The corresponding median values were 281 days and 284 days, respectively. Probably because of more preterm deliveries, women below 20 years of age had a significantly shorter pregnancy length than women 20-34 years of age. Women carrying girls had a significantly shorter pregnancy length than women carrying boys. There was a tendency to shorter pregnancy lengths among multiparae and among smokers.