A prospective study of sex preselection provided the opportunity to characterize the fertile menstrual cycle. We describe from 91 natural conceptual cycles, or sub-groups thereof, cervical mucus symptoms, basal body temperature (BBT) changes, hormonal characteristics, and the outcome of pregnancy. The cervical mucus symptoms defined a potential fertile period of 10 days' average length, with the "peak" mucus symptom occurring at a mean of day 15.5 of the cycle. A fertile period of 9 to 10 days was also indicated by pregnancies resulting from single acts of intercourse between -6 and +3 days from ovulation. The BBT chart was biphasic in 73 of 76 cycles. The duration of the LH surge as observed in early morning urine samples averaged five days, with the peak occurring 1.4 days after the onset. Considerable inter-subject variability was seen in the LH excretion levels. Hormone measurements of peripheral plasma during the luteal phase showed the first detectable presence of hCG between day 7 and day 13 after conception. Progesterone concentrations in the midluteal phase exceeded 20 nmol/L and tended to be higher than in a comparison group of nonfertile cycles, whereas the estradiol concentrations were similar in fertile and nonfertile cycles. The birth sex ratio favored males when intercourse preceded ovulation/fertilization by two days or longer. While this association was statistically significant, the number of pregnancies involved is too small to conclude that the relationship is real.