How Your Due Date Is Calculated (and How Accurate It Is)

The standard calculation used by doctors worldwide is Naegele's rule, published in 1812 by German obstetrician Franz Karl Naegele. Despite its age, it remains the clinical baseline.

The rule works like this:

1. Take the first day of your last menstrual period (LMP).
2. Subtract 3 months.
3. Add 7 days.
4. Add 1 year (if needed to land in the correct calendar year).

Worked example: LMP is August 10, 2025.

1. August 10 → subtract 3 months → May 10
2. May 10 → add 7 days → May 17, 2026

That is the estimated due date (EDD). Equivalently, you can simply add 280 days (40 weeks) to your LMP date — same result, easier arithmetic. The 280-day assumption rests on a typical 28-day cycle where ovulation and conception happen around day 14.

Naegele's rule assumes a textbook 28-day cycle with ovulation exactly at day 14. In practice, cycle lengths range from 21 to 35 days in healthy women, and ovulation can shift by several days even within a regular cycle. This introduces an error of potentially one to two weeks before a single ultrasound is done.

Common sources of LMP-based inaccuracy:

• Irregular cycles — longer or shorter cycles push ovulation earlier or later than day 14.
• Uncertain recall — studies show roughly 20–30% of women are unsure of their exact LMP date.
• Recent hormonal contraceptive use — cycles can take weeks to normalize after stopping the pill, making LMP unreliable.
• Conception timing — sperm survive up to 5 days in the reproductive tract, so intercourse date does not pinpoint fertilization.

Because of these factors, an LMP-only due date without ultrasound confirmation carries a margin of error of roughly ±2 weeks in either direction at the population level.

A first-trimester ultrasound — ideally between 8 and 13 weeks 6 days — measures the fetal crown-rump length (CRL) and uses standardized growth charts to estimate gestational age. Before 14 weeks, all fetuses grow at nearly the same rate regardless of genetics, so CRL is a reliable size proxy for age.

First-trimester ultrasound accuracy: ±5 to 7 days.

Once into the second trimester (14–28 weeks), accuracy drops because genetic variation in fetal size starts to matter. Second-trimester measurements (biparietal diameter, femur length, abdominal circumference) carry an error of ±10 to 14 days. Third-trimester ultrasound is the least accurate for dating, with errors of ±3 weeks or more.

Clinical guideline (per ACOG): if the first-trimester ultrasound EDD differs from the LMP-based EDD by more than 7 days, the ultrasound date takes precedence and the chart is adjusted. If they agree within 7 days, the LMP date is typically kept.

This is the number that surprises most first-time parents. Multiple population studies put the rate of birth on the exact estimated due date at 4–5%. Most births cluster in a window around the due date:

• About 70% of births occur within 10 days either side of the EDD (between 39 weeks and 41 weeks).
• About 90% occur between 37 and 42 weeks of gestation.
• Only roughly 5% of pregnancies go past 42 weeks without medical intervention.

The due date is best understood as the midpoint of a probable delivery window, not a deadline. Obstetrically speaking, any birth between 37 weeks 0 days and 42 weeks 0 days is within the range of normal term. A baby born on 38 weeks exactly is not early by clinical standards; a baby born at 41 weeks 4 days is not significantly overdue.

Gestational age is counted from the LMP, not from conception — which means at the moment of conception you are technically already about 2 weeks pregnant by the clinical calendar. Milestones are defined from that LMP start point:

• First trimester: Weeks 1–13. Organ formation is complete by week 10. The most accurate window for ultrasound dating is in this trimester (crown-rump length before 14 weeks).
• Second trimester: Weeks 14–27. The anatomy scan typically occurs at 18–20 weeks. Dating revisions at this stage are less common because the first-trimester date is usually already confirmed.
• Third trimester: Weeks 28–40+. Growth surveillance takes priority over dating. If no prior ultrasound exists, a late-pregnancy scan can estimate gestational age, but the wide error range means management decisions must be made cautiously.

One practical implication: if you arrive at prenatal care late — say at 22 weeks — and have no prior ultrasound, your provider will date the pregnancy using the second-trimester scan plus LMP, accepting a wider uncertainty range. This is why early first-trimester care establishes the most reliable EDD.

In IVF pregnancies, the date of egg retrieval (or embryo transfer) is known to the day. A day-3 embryo transfer means fertilization occurred approximately 3 days before transfer; a day-5 blastocyst transfer means 5 days before. The resulting gestational age calculation is precise, and the EDD calculated from it is as accurate as any obstetric number gets. No LMP estimate needed, no ultrasound correction required in the first trimester — the embryo age is simply known.

Similarly, if a woman tracked ovulation via LH surge testing or basal body temperature, the conception window narrows to 1–2 days, making the LMP-based calculation considerably more reliable than average.

The pregnancy due date calculator on UtilityApp uses Naegele's rule directly: it takes the LMP date you enter, adds 280 days, and returns the EDD along with week-by-week milestones. If your cycle length differs from 28 days, the tool adjusts the calculation by shifting the expected ovulation day accordingly — so a woman with a 35-day cycle gets a due date pushed 7 days later than the standard formula would give.

The result is a best single-point estimate, not a range. For clinical decisions — especially around induction timing or preterm risk — always rely on the EDD established by your provider using your first-trimester ultrasound. Use this calculator to understand the math, to track milestones week by week, and to plan around the most likely delivery window.