Reliability factors of delivery due dates.

This study attempted to identify the factors that are most important in establishing accurate pregnancy due dates. It quantitated the influence of many factors that affect the duration of pregnancy, as calculated from the last menstrual period, with data from a large prospective study. To limit the influence of maternal constitutional factors, analyses were conducted on the differences between the durations of repeat pregnancies in the same mothers. The mean difference between the calculated durations of successive pregnancies in 4,599 mothers was 18 days. This difference was reduced to 10 days when the pregnancies had been planned and the dates of the last menstrual period had been recorded in the first trimester of pregnancy. Many other factors, including maternal and fetal disorders, significantly altered the length of individual pregnancies but had no significant influence on the overall duration of pregnancies in the study.