Your "fertile window"consist of the 6 days each menstrual cycle when you can get pregnant: The ovulation day and the 5 days before that day. Recent data show that ovulation day can vary, even with a regular period. Enter your LMP and cycle length to find your probability of fertility by day.

There are only 5-6 days in a menstrual cycle when intercourse can result in pregnancy, but these 5-6 days can be different from one to the next month. This time period is known as the “fertile window”. The fertile window comprises the five days before ovulation and the day of ovulation itself. We are calculating the "fertile window coverage", the range of days, not just the 5-6 days of fertility PLUS the % chance to help you get pregnant. What most couples don't realize is that just as the day of ovulation varies from cycle to cycle so does the timing of these six fertile days. There are recently more reliable methods to predict ovulation, and therefore predicting the fertile window is has become more reliable.