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