Are Due Dates Set Too Soon?

The Length of Uncomplicated Human Gestation
Mittendorf, R. et al
Obstetrics & Gynecology, V.75, N.6, June 1990 pp. 929-932.
(From the Dept. of Epidemiology, Harvard School of Public Health; Harvard School of Dental Medicine; St. Margaret's Hospital for Women; and Tufts University School of Medicine, Boston MA)

Abstract

By retrospective exclusion of gestations with known obstetric complications, maternal diseases, or unreliable menstrual histories, we found that uncomplicated, spontaneous labor pregnancy in private-care white mothers is longer than Naegele's rule predicts. For primiparas, the median duration of gestation from assumed ovulation to delivery was 274 days (P=.0003). For multiparas, the median duration of pregnancy was 269 days, also significantly longer than the prediction (P=.019). Moreover, the median length of pregnancy in primiparas proved to be significantly longer than that for multiparas (P=.0032). Thus, this study suggests that when estimating a due date for private-care white patients, one should count back 3 months from the first day of the last menses, then add 15 days for primiparas or 10 days for multiparas, instead of using the common algorithm for Naegele's rule.