Calculate ovulation and fertile days
From the first day of your last period and your cycle length, this calculator determines the fertile window, the likely day of ovulation and the dates of your next six cycles. It works backwards from the luteal phase, which – unlike the first half of the cycle – is almost the same length in nearly every woman.
Why the fertile window lasts six days
The egg itself survives only twelve to 24 hours. The window is longer because sperm do: in fertile cervical mucus they remain capable of fertilisation for up to five days. So the days that count are the ones before ovulation, not after.
The six-day figure comes from a study by Wilcox, Weinberg and Baird (New England Journal of Medicine, 1995): among 221 women planning a pregnancy, not a single conception occurred outside this window. The highest probability was not on the day of ovulation but one to two days before.
Why we count backwards from the luteal phase
Most calculators put ovulation in the middle of the cycle. That is wrong for anyone whose cycle is not 28 days: the second half – from ovulation to the next period – is almost the same length in every woman, twelve to sixteen days. It is the first half that varies. In a 35-day cycle, ovulation happens around day 21, not day 17.
Sources
It works with averages. Your body does not. The result is a starting point for a conversation, not a finding – and it cannot see what an examination would show.
Questions about your result? Arrange an appointment.