Fertile Window Calculator
Pregnancy is only possible during a six-day window in each cycle — the five days before ovulation and the day of ovulation itself. Enter your last period and cycle length to see exactly which dates those are for you, and how the odds change across the window.
Your fertile days, day by day
Per-day figures are the published estimates of Wilcox, Weinberg & Baird (New England Journal of Medicine, 1995), from 221 women whose ovulation day was confirmed by hormone testing: roughly 10% five days before ovulation, 16% four days before, 14% three days before, 27% two days before, 31% one day before and 33% on ovulation day itself — and effectively 0% the day after. They are population averages from a modest sample, not a personal prediction.
This cycle on a calendar
Your next cycles at a glance
Each later cycle assumes the same average length. Real cycles drift by a few days, so treat dates further out as rougher estimates.
DueDateCalc works out your due date, week-by-week timeline and milestones.
Why six days — and why they end on ovulation day
The asymmetry is biology, not a rounding choice. Sperm deposited in fertile cervical mucus can survive and remain capable of fertilising an egg for up to five days. The egg, once released, is fertilisable for only about 12–24 hours. So intercourse five days before ovulation can still lead to pregnancy — the sperm are waiting when the egg arrives — while intercourse the day after ovulation almost never does, because by then the egg is gone.
Why the odds are not flat across the window
In the Wilcox, Weinberg & Baird study (New England Journal of Medicine, 1995), 221 women trying to conceive kept daily records and gave daily urine samples, so the true ovulation day could be pinned down by hormone metabolites rather than guessed from the calendar. The chance of conception from intercourse on a single given day rose from about 10% five days out to a peak of about 33% on ovulation day itself, with a small dip three days before (14%, versus 16% four days before) that is present in the published data — the sample on any single day is modest, so read the overall shape rather than any one number. Every pregnancy in the study came from those six days; none came from the day after ovulation.
These are averages across a group, for one act on that day. Your own odds depend on age, health, timing within the day and plain chance — and having intercourse two or three times spread across the window is generally considered more effective than trying to hit a single "best" day.
If your cycles are irregular
The wider your cycle length varies, the wider your true fertile window is spread — a calendar estimate built on one average length will be off in both directions. Use your shortest and longest recent cycles in the calculator above to bracket the plausible window, and consider ovulation predictor kits, which detect the LH surge 24–36 hours before ovulation, for a signal tied to this cycle rather than your average.