Women can accurately predict the day of ovulation by examining vaginal mucus using the Billings Ovulation Method (BOM). they then can choose to either have sexual intercourse if they wish to conceive or to refrain if they do not want to become pregnant by being able to identify the peak day of fertility and the 3 following days of diminishing fertility. A preovulatory phase of the ovulatory cycle follows menstruation which generally includes a sensation of dryness around the genital area but sometimes no dry days occur. At this point progesterone activates the secretion of a thick sticky mucus into the cervical canal which sperms cannot penetrate. This mucus also serves to guard against infection in the reproductive tract and abdominal cavity. When circulating estrogen increases due to follicular development a fluid clear and stringy mucus replaces the thick mucus. this mucus is like a raw egg white. Under the microscope it has a distinctive fern pattern. Sexual intercourse during this time period may result in pregnancy because this mucus sustains sperm cells in cervical crypts for 48-72 hours. Not all women notice the changes in the mucus however but they can sense the vulva as soft swollen and moist. The final day of this sensation identifies the peak day (day 1) in which ovulation customarily occurs. Even though fertility decreases in the next 3 days conception may occur in days 1-3. Infertility sets in on day 4 and last on average 2 weeks. any mucus now seems sticky and opaque. WHO field trials in El Salvador and 4 other countries confirmed that BOM is >99% effective when couples adhere to the guidelines. Research demonstrated that illiterate couples from low socioeconomic status understand it and apply it successfully.