Calculating a Due Date: Naegele's Rule, Weeks of Pregnancy, and What 'Full Term' Means

The Calculation Step by Step

Step 1: record the first day of the last period, for example January 10, 2026. Step 2: add 280 days. January has 21 days left after the 10th (to Jan 31), then February 28, March 31, April 30, May 31, June 30, July 31, August 31, September 30: counting 21 + 28 + 31 + 30 + 31 + 30 + 31 + 31 + 30 = 263 days to September 30, and 17 more days gives October 17. Date calculators perform this instantly.

Gestational age on any date is the number of days since the LMP divided by 7: on April 4, 2026, the pregnancy would be 84 days, or 12 weeks 0 days.

Trimesters and Key Milestones

Pregnancy is divided into three trimesters: the first trimester through week 13 (days 0 to 97), the second from week 14 to 27, and the third from week 28 until birth. Common milestones include a first ultrasound around 8 to 12 weeks, an anatomy scan at 18 to 22 weeks, and viability, generally considered from about 22 to 24 weeks with intensive care. Full term is defined as 39 weeks 0 days through 40 weeks 6 days, early term as 37 to 38 weeks 6 days, late term 41 weeks, and post-term 42 weeks and beyond.

Only about 4% to 5% of babies are born on their exact due date. Most births occur within two weeks before or after it.

Adjustments: Cycle Length and Ultrasound

Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles are longer, say 35 days, ovulation occurs later, and the due date shifts later by the difference (7 days). If cycles are irregular or the LMP is uncertain, clinicians date the pregnancy by ultrasound, usually based on the crown-rump length measured in the first trimester, which is accurate to within about 5 to 7 days. If the ultrasound estimate differs from the LMP date by more than 7 days before 14 weeks (or by other thresholds depending on gestational age), the due date is usually revised to the ultrasound date.

For pregnancies conceived by IVF, the due date is calculated from the embryo transfer date and the embryo's age.

What a Due Date Means

The due date is a guide. Babies are healthy across a range of 37 to 42 weeks. First pregnancies on average go slightly beyond 40 weeks. Doctors monitor and may recommend induction if pregnancy continues past 41 to 42 weeks, or earlier if medical conditions arise. Plan for birth anytime from week 37 on. For maternity leave, the date is used to plan, but flexibility is needed.

Using a Calculator

Date tools make the arithmetic easy, but provide the information to your care provider: they will confirm dating with an ultrasound. This article is general information and not medical advice.