How is EDD calculated?

Add 280 days (40 weeks) to the first day of the last menstrual period.

Should ultrasound override LMP?

When LMP is uncertain or ultrasound differs substantially, follow institutional / ACOG dating criteria.

EDD & EGA Calculator (from LMP)

Medical context

Calculate estimated due date and current estimated gestational age from the first day of the last menstrual period.

Obstetric teams pair dating and labor scores with ultrasound data and institutional obstetric pathways.

This page explains the EDD / EGA formula, how to interpret the output, known limitations, and peer-reviewed references so you can verify results against primary sources and local protocols.

It is grouped under Obstetrics & Gynecology because pregnancy dating, labor scores, and perinatal tools.

How to use this calculator

Enter the values requested by the EDD / EGA calculator using the units shown beside each field.
Review the calculated result and read the interpretation section before acting on the output.
Check limitations and references if the result will change management, dosing, or disposition.
Use the related calculators section when you need supporting values (for example electrolytes, weight, or complementary risk scores).

Calculator

Last menstrual period (LMP)First day of LMPAs of date

Estimated gestational age

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Enter LMP date

Educational use only. Confirm results with clinical judgment, primary sources, and local protocols. Not a substitute for professional medical advice. Full disclaimer.

Formula

EDD = LMP + 280 days (Naegele). EGA = days from LMP to the as-of date, expressed as weeks + days.

Interpretation

Dating by certain LMP assumes a 28-day cycle with ovulation on day 14. Prefer early ultrasound dating when dates are uncertain.

Limitations

Irregular cycles, recent hormonal contraception, and breastfeeding can make LMP dating unreliable.

Frequently asked questions

How is EDD calculated?: Add 280 days (40 weeks) to the first day of the last menstrual period.
Should ultrasound override LMP?: When LMP is uncertain or ultrasound differs substantially, follow institutional / ACOG dating criteria.
Is the EDD / EGA result enough to make a treatment decision?: No. Calculators support medical decision-making but do not replace examination, shared decision-making, institutional protocols, or consultant input when stakes are high.

References

ACOG Committee Opinion on dating pregnancy
Naegele’s rule (classic obstetric dating)