What is Naegele's rule?

Naegele's rule estimates a due date as 280 days (40 weeks) from the first day of the last menstrual period, assuming a typical 28-day cycle.

Is this as accurate as a doctor's estimate?

An ultrasound dating scan, especially in the first trimester, is generally more accurate than LMP-based calculation alone — use this as a starting estimate.

Health

Pregnancy Due Date Calculator

Estimate your due date and current week of pregnancy from your last period.

First day of last menstrual period

Uses Naegele's rule (280 days from LMP) — a general estimate. Your provider's dating scan is more accurate.

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About this tool

Enter the first day of your last menstrual period to estimate your due date using Naegele's rule (280 days from LMP) and see your current week of pregnancy.

How to use this tool

1
Enter your LMP date
Type the first day of your last menstrual period.
2
Read your estimated due date
See the projected due date, 280 days from that date.
3
Check your current week
See how many weeks pregnant you are as of today.

Frequently asked questions

What is Naegele's rule?: Naegele's rule estimates a due date as 280 days (40 weeks) from the first day of the last menstrual period, assuming a typical 28-day cycle.
Is this as accurate as a doctor's estimate?: An ultrasound dating scan, especially in the first trimester, is generally more accurate than LMP-based calculation alone — use this as a starting estimate.