How Your Pregnancy Due Date Is Calculated: Naegele's Rule and Beyond

What Is Naegele's Rule?

The most widely used method for estimating a pregnancy due date is Naegele's rule, a formula attributed to German obstetrician Franz Karl Naegele and published in the early 19th century. The calculation is straightforward: take the first day of the last menstrual period (LMP), add 280 days (or 40 weeks), and the result is your estimated due date (EDD). In practice, this is often simplified as: subtract three months from the LMP date, then add seven days and one year.

The rule rests on two assumptions. First, it treats the pregnancy as lasting 280 days from the LMP rather than from conception. Second, it assumes ovulation occurs on day 14 of a standard 28-day cycle, so conception is placed 14 days after the LMP begins. For women with regular 28-day cycles, Naegele's rule is a reliable starting point. For everyone else, adjustments are often needed.

Gestational Age vs. Embryonic Age

In obstetrics, there are two ways to measure how far along a pregnancy is. Gestational age counts weeks from the first day of the LMP and is the standard used by healthcare providers worldwide. Embryonic age (also called fetal age) counts from the moment of fertilization, which typically occurs about 14 days after the LMP in a 28-day cycle.

This means there is always approximately a two-week difference between gestational age and embryonic age. When a provider says you are '10 weeks pregnant,' they mean 10 weeks of gestational age — but the embryo itself is only about 8 weeks old. Both measurements are valid; the key is knowing which system is being used. This guide, like most clinical references, uses gestational age throughout.

How Ultrasound Dating Works — and When It Takes Over

A first-trimester ultrasound performed between 8 and 13 weeks plus 6 days of gestation can measure the crown-rump length (CRL) of the embryo. The CRL is the distance from the top of the head to the bottom of the buttocks, and it correlates very closely with gestational age at this stage of development. First-trimester CRL measurement has a margin of error of approximately ±5–7 days, making it the gold standard for pregnancy dating.

According to ACOG guidelines (Committee Opinion 700, 2017), if the ultrasound-based EDD differs from the LMP-based EDD by more than 5 days in the first trimester (or more than 7–10 days in the second trimester), the ultrasound date should take precedence. Once the due date has been established by a first-trimester ultrasound, it should not be revised based on later scans, because fetal size variability increases significantly as pregnancy progresses.

Second-trimester ultrasounds (14–28 weeks) measure several fetal biometric parameters — biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) — and are less precise for dating, with a margin of error of ±10–14 days. They remain useful for detecting growth anomalies but are not the preferred method for establishing an EDD.

Other Methods: IVF, Known Conception Date, and ART

Known Conception Date

If you know the exact day of conception — for example, through timed intercourse or intrauterine insemination (IUI) — add 266 days (38 weeks) to that date to get the EDD. This accounts for the fact that conception occurs approximately 14 days after the LMP, so 266 + 14 = 280 days from the LMP, consistent with Naegele's rule.

IVF Transfer Date

For pregnancies achieved through in vitro fertilization (IVF), the embryo's age is known precisely. For a day-5 blastocyst transfer, subtract 19 days from the transfer date to establish an equivalent LMP, then add 280 days — or simply add 261 days to the transfer date. For a day-3 embryo transfer, add 263 days. IVF pregnancies benefit from the most accurate dating of all, because there is no uncertainty about when fertilization occurred.

Accuracy of Due Dates: Only 4% Born on the EDD

Despite the precision implied by a single-date estimate, only about 4–5% of babies are born on their exact estimated due date. A landmark 2013 study published in Human Reproduction (Jukic et al.) tracked pregnancies from natural conception and found that the median duration was approximately 268 days from ovulation — with a range of nearly five weeks between the shortest and longest pregnancies. The study also found that older maternal age and higher pre-pregnancy progesterone levels were associated with longer pregnancies.

Rather than a single date, think of the due date as the center of a normal distribution. Term pregnancy is defined as 37–42 completed weeks of gestation. ACOG further subdivides this into early term (37–38 weeks), full term (39–40 weeks), late term (41 weeks), and post-term (42+ weeks). Babies born in the full-term window (39–40 weeks) generally have the best outcomes for both short-term and long-term health.

Factors That Can Shift Your Due Date

Trimester Milestones at a Glance

When to Contact Your Doctor or Midwife

A due date is a planning tool, not a medical diagnosis. Always work with your healthcare provider for personalized prenatal care. Contact your provider promptly if you experience any of the following:

This article is for educational purposes only and does not replace professional medical advice. Always consult your obstetrician, midwife, or qualified healthcare provider for guidance specific to your pregnancy.