Sources of Seasonal Variation in Recorded Length of Gestation.

An examination of the birth records of New York City for the years 1957-59 reported by Hewitt (1961) indicated that pregnancies which terminated in the summer months tended to be of shorter duration than those which terminated in the winter. February births showed the longest gestations and August births the shortest, the estimated difference being between one and two days. It seemed important to decide whether this fluctuation had a biological origin, or could be ascribed at least in part (as Hewitt suggested) to some seasonal variation either in obstetric practice (such as the use of oxytocic drugs) or in the accuracy with which gestation length was measured. An obvious first step in pursuing this problem was to decide whether the same phenomenon was observable in another area. The present communication reports the results of such an inquiry.