CalcVitalsFertile Window Calculator
The fertile window is the five days before ovulation plus ovulation day itself — six days when conception is possible, because sperm survive up to five days and the egg only one. The two days before ovulation are the most fertile of all. Enter your cycle and this page maps your window.
How it works
Conception is a meeting with mismatched deadlines. The egg, once released, is viable for perhaps 12–24 hours; sperm, deposited in fertile-quality cervical mucus, can wait up to five days. The fertile window is simply the span where those two facts overlap: intercourse up to five days before ovulation can still result in pregnancy, and the day after ovulation almost never can.
Not all six days are equal. Large prospective studies (the landmark is Wilcox and colleagues in the New England Journal of Medicine) found pregnancy probability climbing across the window and peaking in the final two days before ovulation — around 25–30% per act in younger couples — then falling to near zero the day after ovulation. If timing is the project, those two days are where to aim, and every-other-day coverage of the window is statistically just as good as daily.
Remember what the window is built on: an average cycle. If your cycles vary by more than a few days, widen the window by your variation, and let ovulation tests pin the actual day. Couples under 35 are generally advised to seek a fertility workup after 12 months of well-timed trying, over 35 after 6 months.
The method
Window = ovulation − 5 days to ovulation + 1 day, with ovulation = LMP + cycle − 14. Peak days = ovulation − 2 and − 1, per Wilcox et al. (NEJM 1995) conception-probability data.
Frequently asked questions
Can I get pregnant outside the fertile window?
Essentially no — pregnancy requires an egg, and the egg exists for about a day after ovulation. Pregnancies that seem to come from outside the window are almost always cases where ovulation itself happened on an unexpected day, shifting the whole window.
Is daily intercourse better than every other day?
Within the window, daily and every-other-day give very similar conception rates in studies. Daily can slightly lower sperm counts per act; every other day keeps counts high and coverage complete. Choose the sustainable one.
How long should we try before seeing a doctor?
Under 35: after 12 months of regular, window-timed attempts. 35 and over: after 6 months. Immediately, at any age, if cycles are very irregular, absent, or there is a known reproductive condition.