How the chance is estimated
The chance of conception depends mostly on when sex happens relative to ovulation. A landmark study by Wilcox, Weinberg and Baird, published in the New England Journal of Medicine in 1995, followed couples trying to conceive and measured ovulation with daily hormone tests. Pregnancies only resulted from sex in a six-day window ending on the day of ovulation:
| Day of sex | Chance of conception |
|---|---|
| 5 days before ovulation | about 10% |
| 4 days before | about 16% |
| 3 days before | about 14% |
| 2 days before | about 27% |
| 1 day before | about 31% |
| Ovulation day | about 33% |
The calculator estimates your ovulation date from your last period and cycle length, places each date you enter relative to it, and combines the days: if sex on two days had chances of 27% and 33%, the chance that at least one led to conception is 1 − (0.73 × 0.67) ≈ 51%.
Why the result is an estimate
- Ovulation moves. Even with regular cycles it can shift by several days, so a day that looks “outside” the window might not be.
- Age matters. The figures come from healthy couples; the chance per cycle falls in the late thirties and forties.
- Individual factors — sperm quality, cervical mucus, health conditions — make every couple different.
So a low estimate is not a guarantee, and this calculator is not a method of contraception. If you had unprotected sex and don’t want to be pregnant, emergency contraception works best the sooner it is taken — a pharmacist, sexual-health clinic or doctor can help.
What to do next
A pregnancy test is reliable from the day your period is due, or about three weeks after sex — the calculator shows the date. If you are trying to conceive, the Ovulation Calculator shows your fertile window for the next cycles.