Pregnancy Due Date Calculator
Estimate your due date with Naegele's rule. Enter the first day of your last menstrual period to get your due date, current week, and trimester.
One of the first questions after a positive pregnancy test is 'when is the baby due?' This calculator applies Naegele's rule — the same 280-day method doctors use for a first estimate — to your last menstrual period date. You'll get an estimated due date, how many weeks along you are, your current trimester, and a day count to the big day.
How to use this calculator
- Enter the first day of your last menstrual period (LMP).
- The calculator defaults 'today' to the current date — adjust it if you're checking a different date.
- Click Calculate to see your estimated due date.
- Read your current week, trimester, and days remaining alongside it.
The formula
Naegele's rule: estimated due date = first day of LMP + 280 days (40 weeks). Current week = completed weeks since LMP. Trimesters: weeks 1–13 first, 14–27 second, 28+ third.
Worked example
With a last menstrual period of December 1, 2025, the estimated due date is September 7, 2026 — exactly 280 days later. On March 1, 2026 that pregnancy would be at week 12, in the first trimester, with 190 days remaining.
Frequently Asked Questions
What is Naegele's rule?
Naegele's rule estimates a due date by adding 280 days (40 weeks) to the first day of the last menstrual period. It's the standard first-pass method in obstetrics because the LMP is usually the most reliably known date early on. The 280-day figure assumes a 28-day cycle with ovulation on day 14 — real pregnancies vary, which is why it's called an estimated date.
How accurate is the estimated due date?
It's a good starting estimate but not a prediction — only about 5% of babies arrive on their due date, and most arrive within two weeks on either side. An early ultrasound (usually 8–12 weeks) measures the embryo directly and often refines the date by several days. Your provider will use the ultrasound date if it differs meaningfully from the Naegele estimate.
What are the trimester boundaries?
The calculator uses the common convention: weeks 1–13 are the first trimester, weeks 14–27 the second, and week 28 onward the third. Some sources define them slightly differently (for example, splitting 40 weeks into three equal parts), but the 13/27 split is what most pregnancy guides and providers use in practice.
What if my cycles are irregular or I don't know my LMP?
Naegele's rule assumes a regular 28-day cycle, so irregular cycles make the estimate less reliable — with longer cycles the true due date is usually a bit later. If you don't know your LMP date or your cycles vary a lot, an early dating ultrasound is the better tool. Bring whatever cycle information you have to your first prenatal appointment and your provider will sort it out.
Does 'weeks pregnant' include the two weeks before conception?
Yes — and this confuses almost everyone. Pregnancy is conventionally dated from the first day of the last menstrual period, which is about two weeks before ovulation and conception actually occur. So when the calculator says 12 weeks, conception happened roughly 10 weeks ago. Doctors use this convention because the LMP is observable while the exact conception date usually isn't.