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.
Three ways to estimate your conception date
There are three common ways to work out when you conceived, in order of accuracy:
- From an early ultrasound (most accurate): a first-trimester scan dates the pregnancy within a few days and back-calculates conception. Trust the earliest scan.
- From your due date: subtract 266 days. If you don't have a due date yet, our due date calculator and pregnancy week calculator can work it out first.
- From your last period: add about 14 days (or your cycle length minus 14). If you want to pin down ovulation more precisely, the ovulation calculator estimates your fertile window from your cycle.
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
When did I conceive?
Conception happens at ovulation, roughly 14 days after the first day of your last period in a 28-day cycle - or about 266 days (38 weeks) before your due date. Because sperm can survive up to 5 days, conception could have occurred on any day in the 6-day window ending on ovulation. Enter your due date or last period above to get your most likely conception day and that window.
How do I calculate conception date from my due date?
Subtract 266 days (38 weeks) from your due date - that lands on the approximate conception day. This works because a due date is set at 40 weeks from your last period, and conception happens about 2 weeks into that count. The calculator above does this for you and also shows the fertile window around that date.
How do I calculate conception date from my last period (LMP)?
Add about 14 days to the first day of your last menstrual period for a 28-day cycle. If your cycle is longer or shorter, ovulation - and therefore conception - shifts accordingly: roughly your cycle length minus 14 days after your period starts. Enter your LMP above and the tool adjusts for your cycle length.
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.