Find out when you conceived — from your due date, your baby's birthday, or your last period. Includes a reverse due date calculator and a full due date calculator.
Conception almost always happens within about 24 hours of ovulation. For a typical pregnancy, that lands roughly 266 days (38 weeks) before your due date — or about 11 to 21 days after the first day of your last period. Because sperm can survive up to five days, the calculator shows a conception window rather than a single day: intercourse on any day in that window could have led to conception.
A reverse due date calculator runs the standard due-date math backward. Doctors estimate a due date by adding 280 days to the last menstrual period; conception happens about two weeks into that count. So the calculator takes your due date (or your baby's birthday) and subtracts about 266 days to find the most likely conception date, with a window of a few days on either side. If the baby arrived early or late, the birth-date option lets you adjust for that.
Treat every result as a well-informed estimate, not a certainty. Ovulation day shifts from cycle to cycle, and even an early dating ultrasound — the most accurate method available — is only reliable to within about 5 days. No calculator can pinpoint the exact day of conception, and results here can't determine paternity on their own when possible dates fall close together.
The due date calculator uses the standard method used by doctors and midwives: 280 days (40 weeks) from the first day of your last menstrual period, adjusted for your cycle length. Only about 1 in 20 babies arrive on the exact date — most arrive within two weeks either side. Your provider may adjust the date after a first-trimester ultrasound.
Medical note: This tool is for general information only and is not medical advice, a diagnosis, or a substitute for care from a doctor or midwife. Dates are estimates. Always confirm pregnancy dating with your healthcare provider.