Due Date Calculator
Estimate your baby’s due date from your last period (with your cycle length), conception date, IVF transfer, an ultrasound or a known due date — with how far along you are, your trimester, a milestone timeline and a week-by-week calendar.
How it works
Last period (Naegele’s rule): due date = LMP + 280 days + (cycle length − 28) · conception: + 266 days · IVF transfer: day-3 embryo + 263, day-5 + 261, day-6 + 260 days · ultrasound: scan date − gestational age at the scan + 280 days · gestational age = date checked − dating start, in weeks and days
Every method finds the same anchor: the “dating start”, the first day of a 40-week (280-day) count that begins about two weeks before conception. From a last period, Naegele’s rule adds 280 days and shifts the date by your cycle length minus 28, because a longer cycle moves ovulation later. A conception date is already two weeks into that count, so 266 days are added. An IVF transfer also counts from a known fertilization date: a day-3 embryo was fertilized three days before transfer, so the due date is 263 days after the transfer, and day-5 and day-6 blastocysts get 261 and 260 days. An ultrasound gives the pregnancy length on the scan day, so the dating start is the scan date minus that length. With a scan you can also enter your last period: the calculator applies the American College of Obstetricians and Gynecologists (ACOG) rule and keeps the last-period date unless the two differ by more than 5 to 21 days, depending on how far along you were at the scan. From the dating start it builds a milestone timeline (heartbeat, screening tests, anatomy scan, viability, term dates) and a week-by-week calendar, and shows your gestational age, week and trimester on any date you pick.
Worked example
A last period starting January 1, 2026 with a 28-day cycle: adding 280 days gives a due date of Thursday, October 8, 2026, with conception around January 15. With a 30-day cycle the date moves 2 days later, to October 10. On June 1, 2026 the pregnancy would be 21 weeks 4 days along (week 22, second trimester), with 129 days to go. The anatomy scan window (18–22 weeks) runs May 7 to June 4, and full term (39 weeks 0 days to 40 weeks 6 days) runs October 1 to October 14.
Sources: ACOG Committee Opinion 700 — Methods for Estimating the Due Date · ACOG Committee Opinion 579 — Definition of Term Pregnancy
Frequently asked questions
How accurate is a due date?
It’s an estimate: only about 4% of babies are born on their due date, and most arrive within two weeks either side. An ultrasound before about 14 weeks dates a pregnancy most accurately — always use the date your doctor or midwife gives.
Why 40 weeks when pregnancy is “nine months”?
The 280 days are counted from the first day of your last period, about two weeks before conception, so the count starts before you were actually pregnant. Measured from conception, gestation is closer to 38 weeks, or about nine calendar months.
Does my cycle length really change the date?
Yes, a little. The standard formula assumes a 28-day cycle with ovulation about two weeks in. A longer cycle pushes ovulation — and the due date — later, and a shorter one pulls it earlier, by the difference from 28 days. Only a scan can confirm it.
How is the due date worked out after IVF?
From the transfer date and the embryo’s age: add 263 days for a day-3 embryo, 261 for a day-5 blastocyst and 260 for a day-6 blastocyst. Because the fertilization date is known exactly, this is usually the most reliable calendar estimate.
My scan and my last period give different dates — which one counts?
Turn on the ACOG comparison in ultrasound mode. ACOG guidance keeps the last-period date unless the scan differs by more than 5 days (before 9 weeks), 7 days (9–15 weeks), 10 days (16–21 weeks), 14 days (22–27 weeks) or 21 days (28 weeks on). Your provider makes the final decision.
What do early, full and late term mean?
ACOG uses early term for 37 weeks 0 days to 38 weeks 6 days, full term for 39 weeks 0 days to 40 weeks 6 days, late term for 41 weeks 0 days to 41 weeks 6 days and post-term from 42 weeks. The milestone table shows those dates for your pregnancy.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.