Pregnancy Due Date Calculator

Estimate your baby's due date and conception date from the first day of your last menstrual period (LMP), using the standard Naegele's rule of 280 days from LMP.

First day of last period (LMP)

Estimated due date

July 19, 2027

Conception estimateOctober 26, 2026

Progress today0 weeks, 0 days

TrimesterFirst trimester

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How it works

Naegele's rule adds 280 days (40 weeks) to the first day of your last period. Conception is estimated at around 14 days after LMP.

Due date = LMP + 280 days

Worked examples

Last period started 1 January 2024
An estimated due date of 7 October 2024 - 280 days (40 weeks) counted from the first day of the last menstrual period.
Last period started 10 March 2026
An estimated due date of 15 December 2026 by the same 280-day rule. Because only a small proportion of births fall on the estimated day, antenatal care works to a window around it rather than the date itself.
Forty weeks from the period, thirty-eight from conception
A pregnancy dated at 40 weeks from the first day of the last period is about 38 weeks from conception, because the count starts roughly two weeks before ovulation. Both numbers describe the same estimated date, and the 40-week figure is the one antenatal care uses.

What this assumes, and where it does not apply

What it assumes

Naegele's rule: 280 days (40 weeks) from the first day of the last menstrual period.
Assumes a regular 28-day cycle with ovulation around day 14.
The pregnancy is presumed to be a singleton, because the rule adds a fixed 280 days and is not adjusted for a multiple birth.

Where the result does not apply

A cycle longer or shorter than 28 days shifts the estimate; a dating scan is more accurate and is what your clinic will use.
Not a substitute for antenatal care, and it cannot detect or rule out any complication.
It does not apply when the first day of the last period is not known, which is the one input the rule depends on entirely.

Frequently asked questions

How accurate is the due date?

Only about 4–5% of babies are born exactly on the due date; it is an estimate.

How is pregnancy counted?

Pregnancy is dated from the first day of your last period, about two weeks before conception.

How many weeks is a full-term pregnancy?

A full-term pregnancy is about 40 weeks (280 days) from the last period.

Can I use the due date estimate if my cycle is not 28 days?

The rule assumes a 28-day cycle, so a longer cycle usually moves the estimate later than the date shown and a shorter one earlier. The dating scan is the figure your clinic will use, and it can differ from this estimate.

This tool is for general information only and is not medical advice. Always consult a qualified healthcare professional before making health-related decisions.

Delta Tech Solutions prepared this page. No doctor, nurse or other qualified health professional reviewed it, and it is not medical advice — talk to a qualified healthcare professional about your own situation.

Sources: American College of Obstetricians and Gynecologists

Maintained by David M at Delta Tech Solutions · Figures last checked 10 October 2026 · Next check due 10 April 2027

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