Due Date Calculator - Pregnancy Due Date, Weeks & Trimester

Naegele's rule, the method behind the estimate

The standard way to estimate a due date is Naegele's rule, named after the German obstetrician Franz Naegele, who described it in the early 1800s. Take the first day of your last menstrual period (LMP) and add 280 days, which is 40 weeks. The paper-and-pencil version is to add one year, subtract three months, and add seven days.

The rule assumes a 28-day cycle with ovulation on day 14. If your cycles are consistently longer or shorter, ovulation shifts too, so the calculator moves the due date by the difference. A 32-day cycle adds 4 days; a 25-day cycle subtracts 3.

Worked example

Your last period started on Saturday, January 10, 2026, and your cycles run 28 days. Add 280 days: the estimated due date is Saturday, October 17, 2026. Likely conception was around January 24, two weeks after the LMP.

Same LMP, but your cycles run 32 days. Ovulation probably happened around day 18, so the due date moves 4 days later, to Wednesday, October 21, 2026.

Gestational age is counted from the LMP too. On March 21, 2026, 70 days after January 10, you would be 10 weeks 0 days pregnant, even though conception happened only about 8 weeks earlier. That two-week offset confuses a lot of people, but it is how every provider counts.

How accurate is it?

A due date is the middle of a range, not an appointment. Only about 4 to 5% of babies arrive on the exact day. ACOG defines early term as 37 weeks 0 days to 38 weeks 6 days, full term as 39 weeks 0 days to 40 weeks 6 days, late term as 41 weeks, and post-term as 42 weeks and beyond. First pregnancies run slightly longer on average.

LMP dating is less reliable if your cycles are irregular, you recently stopped hormonal birth control, you are breastfeeding, or you are unsure of the date. In those cases an early ultrasound gives a better answer.

When an ultrasound changes the date

ACOG's guidance is that a first-trimester ultrasound measuring the crown-rump length is the most accurate way to establish or confirm gestational age. If the ultrasound estimate differs from the LMP estimate by more than 5 days before 9 weeks, or by more than 7 days from 9 weeks through 13 weeks 6 days, the due date is changed to match the ultrasound. Later in pregnancy, ultrasound dating becomes less precise, so the thresholds get wider.

For IVF pregnancies, the due date comes from the embryo transfer: 261 days after a day-5 blastocyst transfer, or 263 days after a day-3 transfer. Your fertility clinic will give you that date, and it replaces the LMP estimate.

Trimesters and milestones

First trimester: through 13 weeks 6 days. This is when most people have their first prenatal visit, typically around 8 to 10 weeks, and optional genetic screening. Second trimester: 14 weeks 0 days through 27 weeks 6 days, including the anatomy scan at about 18 to 22 weeks. Third trimester: 28 weeks until birth, with glucose screening usually between 24 and 28 weeks and group B strep testing at 36 to 37 weeks.

The calculator shows your current trimester and how many days remain, so you can map these milestones onto a calendar. The date calculator can add specific weeks to your LMP if you want exact dates for each appointment window.

Common mistakes

Entering the date of conception or a positive test instead of the first day of your last period. The calculator expects the LMP; using conception date pushes everything two weeks late.

Using the last day of your period instead of the first. That can shift the estimate by five days or more.

Planning around the exact date. Treat the two weeks before and after as the real window, especially for travel, leave from work, or family visits.