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.

Can I calculate my due date from the conception date?

Yes. The due date is about 266 days (38 weeks) after conception. With this calculator, enter a last period date 14 days before the conception date and keep the cycle length at 28. For example, conception on January 15, 2026 gives a due date of October 8, 2026.

How is the due date set after IVF?

ACOG Committee Opinion No. 700, "Methods for Estimating the Due Date", dates IVF pregnancies from the embryo transfer: the due date is 266 days after transfer minus the embryo's age, which is 263 days after a day-3 transfer and 261 days after a day-5 transfer. This calculator works from the last period, so use your clinic's date if you had IVF.

Due Date Calculator

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

Your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period, known as Naegele's rule.

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

Quick answer

Your estimated due date is 280 days (40 weeks) after the first day of your last menstrual period, known as Naegele's rule. If your cycle is longer or shorter than 28 days, the date moves by the difference. A last period starting January 1, 2026 with a 28-day cycle gives a due date of October 8, 2026.

Examples

Last period started	Cycle length	Due date
Jan 1, 2026	28 days	Oct 8, 2026
Jan 1, 2026	32 days	Oct 12, 2026
Mar 15, 2026	28 days	Dec 20, 2026

Enter your last period date to see your due date, current week and milestones.

Summary

This due date calculator applies Naegele's rule — the standard obstetric formula — to estimate your delivery date from the first day of your last menstrual period. It supports custom cycle lengths (21–45 days), displays your current gestational age in weeks and days, identifies your trimester, and lists upcoming prenatal milestones with their expected dates. All calculations run entirely in your browser; no personal data is sent anywhere.

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: 2026-05-23 · Reviewed by Nham Vu