[Is the length of gestation a multiple of a fundamental cycle?]

Pregnancy duration was accurately determined using early ultrasonography in 21 patients prior to spontaneous abortion and in two successive pregnancies in each of 57 women prior to spontaneous delivery. Pregnancy durations before abortion or delivery approximated multiples of 14 days (14.4 and 13.3 days, respectively). In addition, in a given woman, the duration of two successive pregnancies carried to term was often identical; in a smaller number of cases, a difference of about 12 days (calculated value, 11.8 days) or, rarely, 24 days, was found. These values may correspond to the same functional period. These observations suggest that the duration of pregnancy (mean, 266 days) may be a multiple of the duration of the luteal phase of the menstrual cycle (12 to 14 days). Interindividual variations in luteal phase duration (standard deviation, 0.5 to 1 day) may explain inter-individual variations in pregnancy duration (standard deviation of about 15 days). The mean multiple may be 20. Thus, pregnancy duration may be determined both by fetal maturation and by a maternal time-counting mechanism. This hypothesis is discussed in the light of current data.