How accurate is an LMP-based due date?

It is an estimate with a margin of error of roughly ±3 weeks. Only about 1 in 20 pregnancies deliver on the EDD, and a normal term spans 37 to 42 weeks. A first-trimester ultrasound is more accurate.

What is dating-scan mode?

If you have a gestational age measured at a dating ultrasound, enter that age and the scan date and the tool carries it forward instead of using the LMP. The tool does not auto-redate — the clinician decides which dating to use.

Does this confirm a pregnancy or assess the baby?

No. It is a calendar calculation only. It does not confirm a pregnancy, assess fetal wellbeing or viability, or replace a dating ultrasound or clinical assessment.

Does any of the data I enter leave my browser?

No. The calculation runs entirely in your browser; nothing you enter is sent to a server or stored. The print option produces a clean dated worksheet you can save as a PDF.

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Free tool

Pregnancy due date & gestational age calculator

Enter the last menstrual period to estimate the due date (EDD) by Naegele's rule, the current gestational age in weeks + days, the trimester, and a ±7-day delivery window — or switch to dating-scan mode and carry a scan forward. A dating aid, not a diagnosis, and nothing you enter leaves your device.

Dating method

First day of last menstrual period

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Cycle length (days)

LMP dating assumes a regular 28-day cycle with ovulation on day 14. This tool does not silently adjust the date for a different cycle length.

Dating estimate

Enter the first day of the last menstrual period to see the estimate.

Dating aid, not a diagnosis or a deadline. The EDD is an estimate computed by Naegele's rule (LMP + 280 days), not a guarantee — only about 1 in 20 babies arrive on the EDD and normal term is 37–42 weeks. The LMP method assumes a regular 28-day cycle with ovulation on day 14 and that the LMP date is recalled accurately (margin of error roughly ±3 weeks). A first-trimester ultrasound (crown–rump length) is more accurate than LMP dating and is used to confirm or revise the EDD — this tool does NOT redate. It recommends no test, drug, monitoring, or management. Runs entirely in your browser — nothing you enter is sent to a server or stored. Not a medical device.

How the due date and gestational age are worked out

Naegele's rule sets the estimated due date 280 days (40 weeks) after the first day of the last menstrual period, and gestational age is simply the completed weeks and days since that date. It assumes a regular 28-day cycle with ovulation on day 14, so a first-trimester dating scan is more accurate and is used to confirm or revise the date.

This calculator mirrors the same dating arithmetic the Avinya Plus EMR runs on every antenatal visit — entered once, the record carries the EDD, gestational age, and trimester forward automatically. If you run an antenatal service, our OB-GYN clinic workflow page shows how that dating sits inside the antenatal record, and the IVF & fertility workflow page covers dating from a known conception or transfer date.

Frequently asked questions

How is the due date (EDD) calculated?: By Naegele's rule: the estimated due date is 280 days (40 weeks) after the first day of the last menstrual period. It assumes a regular 28-day cycle with ovulation on day 14.
How accurate is an LMP-based due date?: It is an estimate with a margin of error of roughly ±3 weeks. Only about 1 in 20 pregnancies deliver on the EDD, and a normal term spans 37 to 42 weeks. A first-trimester ultrasound is more accurate.
What is dating-scan mode?: If you have a gestational age measured at a dating ultrasound, enter that age and the scan date and the tool carries it forward instead of using the LMP. The tool does not auto-redate — the clinician decides which dating to use.
Does this confirm a pregnancy or assess the baby?: No. It is a calendar calculation only. It does not confirm a pregnancy, assess fetal wellbeing or viability, or replace a dating ultrasound or clinical assessment.
Does any of the data I enter leave my browser?: No. The calculation runs entirely in your browser; nothing you enter is sent to a server or stored. The print option produces a clean dated worksheet you can save as a PDF.

Sources

The 280-day rule and trimester boundaries are reproduced from ACOG Committee Opinion 700 and StatPearls (NBK536986). EDD and gestational age are estimates; an early dating ultrasound supersedes LMP-based dating. Guidelines change — verify against current ACOG / RCOG / national guidance.

Carry the dates into the record, not a calculator.

In Avinya Plus the antenatal record computes the EDD, gestational age, and trimester from the LMP automatically and plots them on every visit — no re-keying, no drift. See it on a quick demo.