Physiologic changes during normal pregnancy.

Investigators in the past year have turned their efforts to several age-old obstetric questions. Using modern experimental tools, questions were raised about the validity of Nägele's rule of 280 days as a mean gestational length. One large study agreed with the 280-day mean length, while two other studies disputed it. Current guidelines for appropriate weight gain in pregnancy range from 9 to 14 kg. Several studies found that mean weight gain in healthy pregnant women was greater than these guidelines, with a mean of 15 kg and a normal range from 8 to 25 kg. Women who exercise strenuously throughout gestation were found to have smaller babies than control subjects, with a very mild asymmetric growth retardation. Their labors came on earlier, were shorter in duration, and involved less obstetric and surgical intervention. In an interesting study of maternal hemodynamics, standing was found to be associated with a decrease of more than 15% in cardiac output compared with output in the lateral supine position in the third trimester. Investigators studied exercise in the water and found it to have less effect on fetal cardiovascular parameters than exercise of similar intensity on land.