What is Naegele's rule?

Naegele's rule is the standard obstetric formula: take the first day of the last menstrual period, add 280 days (or equivalently, subtract 3 calendar months and add 7 days). It assumes a 28-day cycle with ovulation on day 14. This calculator automatically adjusts for cycles that differ from 28 days.

How accurate is a due date calculated from the LMP?

LMP-based dating is accurate to within 1–2 weeks for women with regular cycles. Only about 5% of babies are born on their exact due date. An early ultrasound (before 14 weeks) typically provides more precise dating and may revise the LMP estimate.

Why does my cycle length affect the due date?

Standard Naegele's rule assumes a 28-day cycle. If your cycle is shorter or longer, ovulation — and therefore conception — shifts accordingly. For every day your cycle differs from 28, the EDD shifts by one day in the same direction.

When does each trimester begin and end?

The first trimester spans gestational weeks 1 through 12. The second trimester covers weeks 13 through 26. The third trimester runs from week 27 until delivery, with 40 weeks being the standard estimated due date.

What is gestational age vs. fetal age?

Gestational age counts from the first day of the LMP — typically about two weeks before conception. Fetal age (embryonic age) counts from actual fertilization. Healthcare providers use gestational age because the LMP date is easier to establish than the exact conception date.

What should I do if my ultrasound gives a different due date?

If your ultrasound dating differs from the LMP estimate by more than 5–7 days, your provider will typically adjust the official EDD to match the ultrasound, as early scans are more accurate. Use that date as your reference and discuss any discrepancies with your doctor.

Due Date Calculator

Name: Due Date Calculator
Availability: InStock
Author: Nham Vu

Enter the first day of your last menstrual period to instantly calculate your estimated due date, current gestational age, and key pregnancy milestones.

Pregnancy Information

First Day of Last Menstrual Period (LMP)

Enter the first day of your most recent period.

Average Cycle Length (days)

Standard is 28 days. Valid range: 21–45 days.

Naegele's Rule: EDD = LMP + 280 days, adjusted for cycle length deviations from 28 days.

Enter your LMP date and click Calculate

Your estimated due date will appear here

Summary

Enter the first day of your last menstrual period to instantly calculate your estimated due date, current gestational age, and key pregnancy milestones.

How it works

Enter the first day of your last menstrual period (LMP) in the date field.
Adjust the average cycle length if yours differs from the standard 28 days.
Click "Calculate Due Date" to see your estimated due date (EDD).
Review your current gestational age, trimester, and days remaining.
Scroll through the milestones list to see upcoming prenatal checkpoints.

Use cases

Calculate your estimated due date before your first prenatal visit.
Confirm gestational age from the first day of your last period.
Determine which trimester you are currently in.
Plan around key prenatal screenings and ultrasound appointments.
Understand how a non-standard cycle length shifts your due date.
Cross-check an EDD given by your healthcare provider.
Track how many days remain until your estimated delivery date.
Identify the estimated conception window from your LMP.

Frequently Asked Questions

Last updated: 2026-07-01 · Reviewed by Nham Vu