Conception Calculator
Estimate when you conceived from your due date or last period, with the likely conception window and your current gestational age.
If you know your due date — from a scan or an earlier calculation — you can work backward to when you likely conceived. This calculator estimates your conception date and the window around it from either your due date or the first day of your last period, and shows how far along you are today.
How conception dating works
A pregnancy lasts about 266 days (38 weeks) from conception, or 280 days (40 weeks) from the last menstrual period. Conception itself happens around ovulation. So:
- From a due date: conception date = due date − 266 days
- From your last period: conception ≈ ovulation, about (cycle length − 14) days after the period started
Why it’s a window, not a day
You’re fertile for roughly six days each cycle, because sperm can survive up to five days while the egg lives about a day. Fertilisation can therefore occur on any of several days around ovulation. The calculator shows a likely conception window — the five days before through the day after ovulation — alongside the single most-likely date, which is the honest way to present it.
Because of that spread, a conception date cannot identify a specific act of intercourse and is not proof of paternity — only DNA testing can establish that.
Conception vs. gestational age
Pregnancy is dated from the first day of your last period, which is about two weeks before you actually conceive. That’s why you’re considered “about 2 weeks pregnant” on the day of conception, and why 40 weeks from the LMP and 38 weeks from conception describe the same pregnancy. The calculator reports both the estimated conception date and your current gestational age in weeks and days.
A note on accuracy
Estimates from the last period depend on regular cycles and known dates. A first-trimester dating ultrasound is the most accurate way to establish timing, and your clinician may adjust the dates based on it. Use this tool for general understanding, not for medical or legal decisions.
Frequently asked questions
How is the conception date calculated?
Conception happens around ovulation, about 266 days (38 weeks) before the due date. So from a known due date, conception date = due date − 266 days. From your last period, it's estimated at ovulation — about your cycle length minus 14 days after the period started.
How accurate is a conception date?
It's an estimate of a window, not a single certain day. The egg is only fertile for about a day, but sperm can survive up to five days, so conception may have happened on any of several days around ovulation. That's why the calculator shows a likely window as well as a single date.
Can a conception date prove paternity or exact timing?
No. Because the fertile window spans several days, a calculated conception date can't pin down a specific act of intercourse and isn't proof of paternity. Only DNA testing can establish paternity.
Does conception date equal gestational age?
No. Gestational age is counted from the first day of your last period, which is about two weeks before conception. So at conception you're already considered about two weeks pregnant. The calculator shows both the conception date and today's gestational age.