How the due date calculator works
Pregnancy is dated from the first day of the last menstrual period (LMP), about two weeks before conception. A typical pregnancy lasts 280 days (40 weeks) from the LMP. This is Naegele's rule, adjusted for your cycle length:
The other methods convert to the same scale:
IVF: due = transfer date + 266 − embryo age (day 5 → + 261 days, day 3 → + 263)
Ultrasound: due = scan date + 280 − gestational age at scan (in days)
Gestational age today is the number of days since the LMP-equivalent date. Trimesters follow ACOG: first through 13 weeks 6 days, second from 14 weeks to 27 weeks 6 days, third from 28 weeks. Full term is 39 weeks 0 days to 40 weeks 6 days.
A first-trimester ultrasound is the most accurate way to date a pregnancy. If it differs from your LMP date by more than about 5 to 7 days, your provider will usually go by the ultrasound.
Worked example
Your last period started on June 1, 2026 and your cycle is 28 days:
Conception most likely happened around June 15, 2026 (LMP + 14 days). The first trimester ends on September 6, 2026 (13 weeks 6 days), the third trimester starts on December 14, 2026 (28 weeks), and full term runs from March 1 to March 14, 2027. With a 32-day cycle, ovulation is about four days later, so the due date moves to March 12, 2027.
IVF check: a day-5 embryo transferred on June 20, 2026 gives June 20 + 261 days = March 8, 2027.
Frequently asked questions
How is a due date calculated?
Most providers add 280 days (40 weeks) to the first day of your last menstrual period. This is called Naegele's rule. If your cycles are longer or shorter than 28 days, the date shifts by the difference, because ovulation happens later or earlier.
How accurate is my due date?
It is an estimate. Only about 4% of babies are born on their due date, and most arrive within two weeks before or after it. A first-trimester ultrasound measurement is the most accurate way to date a pregnancy.
Why is pregnancy counted from my last period and not conception?
The first day of the last period is a date most people know, while the exact day of conception usually is not. Counting from the period means you are already about two weeks 'pregnant' at conception.
How do I calculate my due date after IVF?
Add 266 days to the transfer date and subtract the embryo's age. For a day-5 blastocyst that is 261 days after transfer, and for a day-3 embryo it is 263 days. Your fertility clinic will confirm the date.
What if my ultrasound date is different from my LMP date?
Providers usually change the due date to match an early ultrasound when the two differ by more than about 5 to 7 days in the first trimester. Choose 'Ultrasound date' and enter the gestational age from your scan report to use it.
When do the trimesters start?
Following ACOG, the first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days to 27 weeks 6 days, and the third from 28 weeks 0 days until birth.
Updated October 2026. Results are estimates for education, not financial or medical advice. All math runs in your browser; nothing you type is sent anywhere.