The length of the fertile window is associated with the chance of spontaneously conceiving an ongoing pregnancy in subfertile couples

BACKGROUND: The period in each menstrual cycle during which sexual intercoursecan result in conception is called the ‘ fertilewindow’. Although the fertilewindow closes on the day of ovulation, little is known about the moment it opens. We defined the first day of normal sperm– mucusinteraction as the opening of the fertilewindow. We hypothesized that length of the fertilewindow varies between couples and that the number of days the fertilewindow is ‘open’ is related to the time to spontaneous conception. METHODS: Serial post-coital tests and sperm– mucus penetration testswere performed to detect the first normal sperm– mucusinteraction day. Ovulation was confirmed by serial ultrasound. Using Cox’ regression analysis, we determined whether the fertilewindow length was associated with time to ongoing pregnancy. This association was expressed in fecundability ratios (FR). RESULTS: The fertilewindow length was determined in 410 subfertile couples. The fertilewindow length varied among couples from 5 days. The FR increased with increasing fertilewindow length and varied between 0.11 (95% CI: 0.03–0.45) for a fertilewindow of 1 day, to 2.4 (95% CI: 1.1– 5.2) for a fertilewindow of 5 days or more. CONCLUSIONS: The longer the fertilewindow in subfertile couples, the higher is the probability of spontaneously conceiving an ongoing pregnancy.