When did conception happen?
Conception usually happens within a day of ovulation, about two weeks after the first day of the last period. Working backward from a due date or a birth date gives the most likely conception date, though not the exact day, since sperm can survive for up to 5 days before the egg is released.
The formulas
- From a due date: conception ≈ due date − 266 days (38 weeks)
- From a birth date: conception ≈ birth date − (pregnancy length in days − 14)
- Likely intercourse window: from 5 days before the conception date to 1 day after
Worked example
Your due date is April 2, 2027.
- Due date − 280 days gives an equivalent last period of June 26, 2026.
- Conception, 14 days later: Friday, July 10, 2026.
- Likely intercourse window: July 5 to July 11, 2026.
For a baby born on April 2, 2027 at 39 weeks, the pregnancy lasted 273 days from the last period, so conception was around July 17, 2026.
Why the date is uncertain
- Ovulation does not always happen on day 14, and cycles vary.
- Sperm can live up to 5 days, so intercourse several days before ovulation can lead to pregnancy.
- Due dates themselves are estimates; an early ultrasound narrows them to about 5 to 7 days.
- Babies are born across a range of several weeks around the due date.
For these reasons, the calculator gives a window, and no date calculation can prove when a pregnancy started. Questions of paternity can only be settled by a DNA test.
Pregnancy timeline after conception
| Event | Time after conception |
|---|---|
| Fertilization | within 24 hours of ovulation |
| Implantation | 6 to 12 days |
| Positive home test | about 2 weeks, at the missed period |
| Heartbeat on ultrasound | about 4 weeks (6 weeks of pregnancy) |
| Due date | 38 weeks (40 weeks of pregnancy) |
IVF pregnancies
With in vitro fertilization, the date of fertilization and transfer is known precisely. Use the IVF due date calculator, which accounts for the age of the embryo at transfer.
A note on medical advice
This calculator gives estimates based on averages. It does not replace advice from your doctor, midwife or health care provider, who will date your pregnancy and follow it with exams and ultrasounds.