Pregnancy Due Date Calculator

How It Works

Doctors date a pregnancy from the first day of the last menstrual period (LMP), not from conception, because the period date is usually the one people remember. Naegele’s rule — the standard method since the early 1800s — adds 280 days (40 weeks) to the LMP to estimate the due date. If you know the conception date instead, the due date is 266 days (38 weeks) after it, since conception typically happens about two weeks after the period starts.

The rule assumes a 28-day cycle: a 32-day cycle moves the due date four days later, a 25-day cycle three days earlier. Trimesters follow from the LMP too — the second begins around week 14, the third around week 28.

Worked Example

With a last period starting on 1 January 2026 and a 28-day cycle, the due date is 1 January + 280 days = 8 October 2026. The estimated conception date is 15 January 2026 (LMP + 14 days), the second trimester begins around 2 April 2026 and the third around 9 July 2026. Counting from a conception date of 15 January gives the same due date: 15 January + 266 days = 8 October 2026.

How to Use This Calculator

Choose whether to calculate from your last period or a conception date, enter the date, and set your average cycle length if you know it. You get your estimated due date, the estimated conception date, trimester start dates and how far along you are today.

Tips and Practical Advice

Treat the due date as a mid-point, not a deadline: most babies arrive within the two weeks before or after it. An early ultrasound often refines the date, especially if your cycles are irregular — your care provider’s dating takes priority over any calculator.

Source: Naegele’s rule (280 days from LMP; 266 days from conception). Reviewed Nov 13, 2026.

Frequently Asked Questions