How conception timing works
Conception requires sperm to meet an egg in the fallopian tube within about 24 hours of ovulation. Because sperm can survive in the reproductive tract for up to 5 days, the 6-day window ending on ovulation day is when conception is possible.
Working backward from a due date: pregnancy lasts about 40 weeks from LMP, or 38 weeks (266 days) from conception. So conception was approximately 266 days before the due date - give or take a few days for the fertile window.
Why estimates differ from "official" dating
Doctors date pregnancy from LMP rather than conception. This means "8 weeks pregnant" really means "about 6 weeks since conception." The extra 2 weeks are baked into the convention. Don't be confused if a doctor says one number and a conception calculator gives a different "actual age."
Frequently asked questions
How is the conception date calculated?
Conception happens at ovulation. If you know your due date, conception was about 266 days earlier (40 weeks − 2 weeks of pre-conception cycle = 38 weeks from conception to birth). If you know your last period (LMP), conception happened around 14 days after that - adjusted if your cycle isn't 28 days.
Can you tell the exact conception date?
No method gives an exact date unless you tracked ovulation precisely with BBT or OPK during that specific cycle. The calculator gives the most likely day, plus a 6-day window because sperm can survive up to 5 days in the reproductive tract - so conception could have happened from any sex within that window.
How is conception date different from LMP?
Pregnancy is dated from the first day of your last menstrual period (LMP), not from conception. Conception happens about 14 days after LMP in a 28-day cycle. So when a doctor says you're 8 weeks pregnant, the embryo is actually about 6 weeks old - the extra 2 weeks come from counting from LMP.
Does the conception date matter for paternity?
If multiple potential fathers are involved within the same cycle, a calculator can narrow the window but not identify a single father with certainty - especially given the 6-day fertile window. Definitive paternity requires DNA testing. Non-invasive prenatal paternity testing (NIPP) is available from about 8 weeks of pregnancy.
What if my cycle is irregular?
For irregular cycles, ovulation timing is harder to pin down. A first-trimester ultrasound (especially before week 13) is the most accurate way to date a pregnancy and back-calculate conception. The earlier the scan, the more accurate the dating.
How accurate is conception dating from an ultrasound?
First-trimester ultrasound (weeks 8–13) dates pregnancy within ±3–5 days. Second-trimester scans are accurate within ±1–2 weeks. Third-trimester scans are less reliable for dating because baby size varies more by then. Always trust the earliest ultrasound for the most accurate conception estimate.
These calculators give estimates based on cycle averages and standard formulas. They are for general information only and are not medical advice. For anything concerning your health or pregnancy, talk to a qualified healthcare provider.