How to Calculate Your Pregnancy Due Date

Guides · Pregnancy · Updated 2026

Finding out you're pregnant brings a flood of questions, and one of the first is "When will my baby arrive?" The estimated due date (EDD) gives you a timeline to plan maternity leave, prenatal visits, and that much-anticipated countdown. While no calculation can predict the exact day, a proven formula — Naegele's rule — has been helping parents and healthcare providers for over a century. In this guide, you'll learn how to calculate your due date from your last menstrual period (LMP) and understand why the date is an estimate, not a deadline.

Why the Due Date Is Just an Estimate

The due date marks 40 weeks from the first day of your last period, but very few babies stick to that schedule. Studies show only about 4-5% of women deliver exactly on their EDD. Most first-time mothers go past the due date, and many factors — from cycle length to ovulation timing — shift the true due date. Think of it as a window (38 to 42 weeks) rather than a single day. The tool below helps you understand that window better.

• Ovulation date is often unknown, causing variation.
• Only 4% of deliveries occur on the exact EDD.
• First babies tend to arrive later than predicted.

Step-by-step: Calculate Your Due Date

1. Open the Pregnancy Due Date Calculator tool.
2. Enter the first day of your last menstrual period (LMP). Be as accurate as possible — even a few days can shift the EDD by a week.
3. If you know the date of conception (ovulation/fertile window), you can also input that instead for a more precise estimate.
4. Click calculate. The tool applies Naegele's rule (LMP + 280 days or LMP minus 3 months plus 7 days) and displays your estimated due date along with the start of each trimester.

How Naegele's Rule Works

Naegele's rule, developed in the 19th century, assumes a 28-day menstrual cycle with ovulation on day 14. The formula adds 280 days (40 weeks) to the LMP. A quick mental version: take your LMP date, add one year, subtract three months, then add seven days. For example, if LMP is 10 April 2026, subtract three months → 10 January 2027, add seven days → 17 January 2027. The tool does this instantly, even accounting for cycle length variations.

Frequently Asked Questions

What if I don't know my LMP date?

If LMP is uncertain, an early ultrasound (before 12 weeks) is the most reliable dating method. You can also use the conception date if you tracked ovulation. The tool accepts both inputs.

Does the due date change if I conceived via IVF?

Yes, IVF due dates are calculated from the embryo transfer date and the embryo's age (day‑3 or day‑5). Our tool allows a conception‑date input for that scenario.

Why do doctors use the LMP and not conception date?

LMP is easier to recall than the exact ovulation day. Since the first day of your period is a clear marker, it became the standard reference point for pregnancy dating worldwide.

Is a "post‑term" pregnancy (past 42 weeks) risky?

Most providers monitor closely after 41 weeks because placental function can decline. Induction is often discussed. The exact management should be decided with your doctor.

Is it free and private?

Yes — the tool runs entirely in your browser, free, with no sign‑up and nothing uploaded to a server.

This guide is for informational purposes. Please consult your doctor or midwife for personalized pregnancy care and dating.