How to Calculate a Due Date (EDD) and Gestational Age

To estimate a due date from the last menstrual period (LMP), add 280 days to the first day of that period, or use Naegele’s rule: add 7 days, subtract 3 months, add 1 year. Gestational age is the time since the first day of the LMP, written in weeks and days. An early ultrasound can confirm or change the LMP-based date.

The formula

Both ways of finding the estimated due date (EDD) start with the first day of the last menstrual period. The average pregnancy is described as 280 days from the first day of the LMP, which is 40 weeks.

• Counting method: EDD = LMP + 280 days.
• Naegele’s rule: EDD = LMP + 7 days - 3 months + 1 year. The StatPearls article on the estimated date of delivery describes it as subtracting 3 months and adding 7 days.
• Gestational age: days from the LMP to the date of interest, divided by 7. The whole number is the weeks and the remainder is the days. Clinicians write it as a fraction of 7, so 22 0/7 means 22 weeks and 0 days.

Both methods assume a 28-day cycle with ovulation on day 14. They also assume the person is certain of the LMP date.

Because months are different lengths, the two methods can land a day or two apart. The worked example below shows this.

Using it in NurseTools

The EDD and Gestation tool is in clinical review and is not available yet. It is locked in the app and will not be available at launch. NurseTools itself is coming soon to the App Store, and nothing is downloadable today.

The app shows a release-gates timeline: source and rights audit, clinical editorial review, independent testing, and regulatory and App Store review. A tool that has not passed all of these stays locked. No payment or setting unlocks a tool that has not passed review. You can turn on the “Notify me when available” switch to hear about it.

When the app is out, it will not need an account, will never ask for patient data and will work offline. Only the BMI and MAP tools are planned for launch.

Worked example

This is a made-up practice example, not a patient.

Suppose the first day of the last menstrual period is March 12, 2026.

1. Counting method. March 12 + 280 days = December 17, 2026.
2. Naegele’s rule. March 12 + 7 days = March 19. Subtract 3 months: December 19, 2025. Add 1 year: December 19, 2026.
3. Compare. The two answers differ by 2 days because of how many days each month has. Many references teach Naegele’s rule for hand calculation, so check which method your course or facility uses.
4. Gestational age on May 7, 2026. From March 12 to May 7 is 56 days. 56 / 7 = 8 with a remainder of 0, so the gestational age is 8 0/7 weeks.
5. Compare with an ultrasound. Say a scan on May 7 measures a CRL that matches 7 2/7 weeks. The difference is 8 0/7 minus 7 2/7, which is 5 days. In the redating table, a scan before 9 0/7 weeks keeps the LMP date if the difference is 5 days or less. The LMP-based date stays.
6. A different result. If the CRL had matched 6 5/7 weeks, the difference would be 9 days, which is more than 5. The ultrasound date would be used instead: 280 days minus 47 days (6 weeks 5 days) is 233 days, and May 7 + 233 days = December 26, 2026.

Limits and common pitfalls

• Uncertain LMP. The LMP method requires that the person is certain of the first day of the last period. The rule also cannot tell menstrual bleeding from early pregnancy bleeding.
• Cycle length. Cycles shorter or longer than 28 days are not accounted for.
• Ultrasound is the stronger tool early. A first-trimester ultrasound is the most accurate method for setting or confirming gestational age, with an accuracy of about 5 to 7 days.
• Redating thresholds depend on gestational age. From the same table: 9 0/7 to 13 6/7 weeks uses 7 days, 14 0/7 to 15 6/7 uses more than 7 days, 16 0/7 to 21 6/7 uses more than 10 days, and 22 0/7 to 27 6/7 uses more than 14 days.
• Date once, then keep it. ACOG says the EDD should be determined, discussed and documented clearly once LMP or first accurate ultrasound data are available, and later changes should be rare.
• Late dating is weaker. ACOG describes a pregnancy without an ultrasound that confirms or revises the EDD before 22 0/7 weeks as suboptimally dated.
• Calculators support, not replace. Follow your facility’s policy and the provider’s documented EDD.