A prospective multicentre trial of the ovulation method of natural family planning. III. Characteristics of the menstrual cycle and of the fertile phase.

Seven hundred twenty-five women of proven fertility recorded the presence of cervical mucus at the vulva in 7514 menstrual cycles. The mean cycle length of the 6472 "normal" cycles was 28.5 days (standard deviation +/- 3.18). The peak day of mucus discharge was the last day of slippery, raw-egg-white-like mucus and occurred on average on day 15 (+/- 2.6). The fertile period was defined as any day on which mucus was reported before the peak day until 3 days after the peak. Its mean length was 9.6 (+/- 2.6) days. The probability of pregnancy was maximal on the peak day and declined on the days before and after the peak.