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Pregnancy Due Date Calculator

Estimate your due date and current pregnancy week using Naegele's rule.

How it is calculated

The tool uses Naegele's rule: due date = first day of last menstrual period (LMP) + 280 days, which is 40 weeks. Pregnancy weeks are counted from the LMP, not from conception, so the gestational age is about two weeks ahead of the actual age of the embryo.

It also compares your LMP with today's date to show the current week (whole days since LMP / 7, rounded down), the days left until the due date (never below 0) and the trimester: weeks 0 to 12 are the first, 13 to 26 the second and 27 onward the third. The rule assumes a regular 28-day cycle and ovulation around day 14. Longer, shorter or irregular cycles shift the real date, and only a small share of babies arrive exactly on the due date. It is an estimate, not medical advice.

Worked example

Say the first day of the last period was 2026-03-01. Adding 280 days (40 weeks) moves the date forward through the rest of the year to December 6, 2026. That is the estimated due date. The current week and days remaining depend on the date you open the tool: each full 7 days since March 1 adds one week, and the trimester switches at the start of week 13 and week 27.

Try this example →

Common mistakes

  • Entering the date of conception or of the positive test instead of the first day of the last period.
  • Using the last day of the period; the count starts on day one of bleeding.
  • Ignoring a cycle that is clearly longer or shorter than 28 days, which makes the date less accurate; an early ultrasound can correct it.
  • Treating the due date as an exact delivery day; babies are born in a window of weeks around it.

About this calculator

This calculator estimates an expected delivery date by starting from the first day of your last menstrual period (LMP) and adding 280 days (40 weeks) — Naegele's rule, the standard convention in obstetric dating. Because actual delivery timing varies naturally from one pregnancy to the next, the result is a statistical estimate centered around the most likely date rather than a fixed prediction.

Expecting parents use it early on to get a general sense of timeline for planning purposes, from figuring out which trimester they're in to booking time off work or preparing a nursery. It is often used alongside ultrasound dating, which can refine the estimate as the pregnancy progresses.

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