Conception Date Calculator
Work backwards from a due date (or last period) to find the likely conception date and the fertile window when conception probably happened.
For general information only — not medical advice. Results are estimates; talk to a doctor or qualified health professional about your own health.
How to use
- 1Enter your due date (or the first day of your last period).
- 2See the most likely conception date and the range around it.
How it's calculated
Conception ≈ due date − 266 days (38 weeks), which is about 14 days after the last period for a 28-day cycle. Most conceptions fall within a few days either side.
Frequently asked questions
Can I know the exact day I conceived?
Usually not. Sperm live up to 5 days, so intercourse up to 5 days before ovulation can lead to conception.
Is the due date from my scan more accurate?
Yes — a first-trimester ultrasound is the most accurate way to date a pregnancy.
Why is pregnancy 40 weeks if conception is at week 2?
Pregnancy is counted from the last period, about two weeks before conception.