Channel formation in dried cervical mucus and the presence or absence of fern formation are correlated. The mean number of channels was 9.2 when the fern was negative, 42.8 when the fern was poor positive, and 73.9 when the fern was good positive. The difference between the means in the 3 groups was statistically significant. The channel number in dried cervical mucus and the presence or absence of fern formation arc correlated with the day of the menstrual cycle in which the specimens were obtained and with the woman's observation of the PEAK mucus symptom while using the ovulation method of natural family planning. The data organized itself more clearly when it was related to the woman's observation of the PEAK symptom. This has been explained by the fact that the PEAK mucus symptom is essentially an ovulatory event. The reliability of the woman's vulvar observations while using the ovulation method was tested against observations of cervical mucus obtained from the level of the endocervix. In those vulvar observations that are considered fertile by the method, the channel number and the frequency of good positive ferning were higher than in those observations that are considered infertile. When the vulvar observations that indicate fertility were compared with those vulvar observations that indicate infertility, the difference was statistically significant. Finally, a practical model of human fertility is described using practical experience with the ovulation method, the woman's vulvar observations while using the method, and the channel and fern formation of endocervical mucus. A progressively declining index of fertility is described that is statistically significant.