Free Pregnancy Tool
Date of Conception
Work backward from your due date or your last period to estimate when your baby was likely conceived.
General information only, not medical advice. Ask your doctor or midwife.
How this works
Formula: Conception ≈ due date − 266 days, or last period + (cycle length − 14) days.
- A pregnancy is about 266 days (38 weeks) from conception to due date, so the calculator counts back 266 days from your due date.
- From your last period, it adds your cycle length and subtracts 14 days, which is when ovulation usually happens.
- Sex in the 5 days before ovulation, or on the day itself, can lead to conception, so the likely window is shown too.
Everything is worked out in your browser. Belly Weekly does not send or store your answers.
Questions
Can this tell me exactly which day I conceived?
No. It gives an estimate. Sperm can live for days, so the sex that led to conception may have been up to 5 days earlier.
Which is more accurate: due date or last period?
A due date set by an early ultrasound is usually the most accurate starting point.
Why is pregnancy called 40 weeks if conception is at week 2?
Pregnancy is counted from the first day of your last period, about 2 weeks before conception, so 40 weeks of pregnancy is about 38 weeks from conception.