Health & Fitness · Pregnancy
Conception Date Calculator — When Did I Conceive?
Work out the probable conception date from a due date (or a baby's birth date), the first day of the last period, or an IVF embryo transfer. See the fertile window when conception most likely happened, the matching due date and, during pregnancy, how far along you are.
A ready-made version of the Pregnancy Due Date Calculator — Week by Week.
How this is calculated
Due date = last period + 280 days + (cycle − 28) · = conception + 266 · = IVF transfer + 266 − embryo age (day 3 → 263, day 5 → 261)
- Gestational age
- Time since the first day of the last period (or the equivalent date for conception/IVF dating)
- Term (ACOG)
- Early term 37⁺⁰–38⁺⁶ weeks, full term 39⁺⁰–40⁺⁶, late term 41⁺⁰–41⁺⁶, post-term from 42⁺⁰
A pregnancy lasts about 280 days (40 weeks) from the first day of the last period — Naegele's rule. Longer or shorter cycles shift ovulation, so the date moves by the difference from 28 days. With a known conception or IVF transfer date the estimate is more precise: 266 days from fertilisation, so an embryo transferred at day 5 has 261 days to go.
Assumptions & sources
- Singleton pregnancy; dates in your local calendar.
- Ultrasound dating, when available, takes precedence.
- 1Methods for Estimating the Due Date (Committee Opinion No. 700) — American College of Obstetricians and Gynecologists
- 2Definition of Term Pregnancy (Committee Opinion No. 579) — American College of Obstetricians and Gynecologists
- 3How to calculate due date from embryo transfer date — San Diego Fertility Center
Calculator ID HLTH-PREG-DUE-001/conception · version 1.1.0
Common questions
How do I work out my conception date from my due date?
Count back 266 days (38 weeks) from the due date — that's the most likely conception date.
Can I find the conception date from a birth date?
Roughly: 266 days before the birth if the baby arrived on time. Babies born early or late shift that estimate.
How accurate is a conception date?
Within a few days at best — ovulation varies and sperm can survive up to five days. An early ultrasound gives the most accurate dating.