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Pregnancy Due Date Calculator

Estimate a pregnancy due date from the last menstrual period, with conception date, current week, and trimester milestones.

Results update instantly.Change any value and the estimate recalculates on the spot.

Health

Pregnancy due date

Estimate a due date from the first day of the last menstrual period.

days

Estimated due date

-

  • Estimated conception date-
  • Current progress-
  • Trimester-

An estimate only - ultrasound dating is more accurate. This is not medical advice. Discuss your pregnancy with a qualified clinician.

How to use this calculator

1. Enter the first day of your last menstrual period. 2. Adjust the average cycle length if yours differs from 28 days. 3. Read the estimated due date, conception date, and current week.

Worked example

Last period started January 1, 2026, with a 28-day cycle: the estimated due date is October 8, 2026, and the estimated conception date is January 15, 2026.

How the math works

Naegele's rule adds 280 days (40 weeks) to the first day of the last period, then adjusts by the difference between your cycle length and 28 days. Gestational age counts from the period date, not conception, which is why pregnancy is described as 40 weeks even though conception happens about two weeks later.

How accurate is the due date estimate?
It is an estimate. Only about 1 in 20 babies arrives on the exact due date. A first-trimester ultrasound gives a more accurate date.
Why does pregnancy count 40 weeks from the period?
By convention, gestational age starts on the first day of the last menstrual period, roughly two weeks before conception. That is why a full-term pregnancy is described as 40 weeks.
What if my cycles are irregular?
The estimate assumes a regular cycle. With irregular cycles, early ultrasound dating is much more reliable than a calendar calculation.
When do trimesters change?
Roughly: weeks 1-13 are the first trimester, weeks 14-27 the second, and week 28 onward the third.

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How it works

How the Pregnancy Due Date Calculator works

Quick answer: The estimated due date is 280 days (40 weeks) after the first day of the last menstrual period, adjusted for cycle length. A last period starting March 1, 2026 with a 28-day cycle gives a due date of Dec 6, 2026.

Formula

Due date = LMP + 280 days + (cycle length − 28)
Estimated conception = LMP + 14 days + (cycle length − 28)

This is Naegele's rule with a cycle-length adjustment.

Worked example

Last period March 1, 2026, 28-day cycle: due date Dec 6, 2026, estimated conception Mar 15, 2026.

Assumptions and limits

  • Only about 1 in 20 babies arrive on the due date; an ultrasound dating scan is more accurate.
  • This is an estimate, not medical advice.

Frequently asked questions

How accurate is the due date estimate?

It is an estimate. Only about 1 in 20 babies arrives on the exact due date. A first-trimester ultrasound gives a more accurate date.

Why does pregnancy count 40 weeks from the period?

By convention, gestational age starts on the first day of the last menstrual period, roughly two weeks before conception. That is why a full-term pregnancy is described as 40 weeks.

What if my cycles are irregular?

The estimate assumes a regular cycle. With irregular cycles, early ultrasound dating is much more reliable than a calendar calculation.

When do trimesters change?

Roughly: weeks 1-13 are the first trimester, weeks 14-27 the second, and week 28 onward the third.

How accurate is a due date from the last period?

It is a good starting estimate. Most births happen within two weeks either side of it.

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