Cervical mucus secretions on the day of intercourse: an accurate marker of highly fertile days.

OBJECTIVE: To provide estimates of the probabilities of conception according to vulvar mucus observations classified by the woman on the day of intercourse. STUDY DESIGN: Prospective cohort study of 193 outwardly healthy Italian women using the Billings Ovulation Method. Outcome measures include 161 conception cycles and 2594 non-conception cycles with daily records of the type of mucus and the occurrences of sexual intercourse. RESULTS: The probability of conception ranged from 0.003 for days with no noticeable secretions to 0.29 for days with most fertile-type mucus detected by the woman. The probability of most fertile type mucus by day of the menstrual cycle increased from values <20% outside of days 10-17 to a peak of 59% on day 13. CONCLUSION: Regardless of the timing of intercourse in the menstrual cycle, the probability of conception is essentially 0 on days with no secretions. This probability increases dramatically to near 30% on days with most fertile-type mucus, an association that accurately predicts both the timing of the fertile interval and the day-specific conception probabilities across the menstrual cycle.