We have analysed the duration of pregnancy for singleton births in Sweden during 1976-80 by means of data from the Swedish Birth Registry. Information, which was obtained from special forms with standard questions, include date of first day of last menstrual period (LMP) and whether that date was considered reliable or not. Recording was done prospectively, starting at the first antenatal visit. In 10% of cases the dates were labelled uncertain. Information on LMP and birth dates, parity, age of mother, sex of child, and/or mode of delivery was missing in 5.5% of the singleton cases, leaving 427,581 singleton births for analysis. In cases of reliable menstrual dates, the average duration from LMP to vaginal birth was 282 days (median), 281 days (mean) and 283 days (mode), remaining constant over the years of study. One standard deviation of the mean was approximately 13 days, varying slightly with age and parity. Ten per cent of these women gave birth post term (past 294 days). The duration of cesarean section births became shorter over the years, in spite of little change in cesarean section frequency (9.5% in 1976-7 and 10.9% in 1979-80). Mothers aged 35 and over tended to give birth 2 days earlier than those below 35. Second and subsequent children of mothers below 35 had slightly shorter gestations than first-borns. Boys were born earlier than girls, on average. When LMP was unreliable, the distribution of gestational lengths was wide. We also noted a seasonal rhythmicity in average duration of pregnancy, with consistent shortening in the month of December.