How Due Dates Are Calculated: Naegele's Rule and Why Estimates Aren't Guarantees

A pregnancy due date is the single most-asked-for number in obstetrics, and yet it is one of the most misunderstood. Your due date is not a deadline or an appointment — it is a statistical estimate, the midpoint of a normal range. Here is how that number is actually produced and why it varies so much in practice.

Naegele’s Rule: the default formula

The standard method, named after the 19th-century German obstetrician Franz Naegele, is:

Due date = first day of the last menstrual period (LMP) + 280 days (40 weeks)

This formula assumes a 28-day menstrual cycle with ovulation on day 14. By counting from the period rather than from conception, it works even when the precise day of fertilization is unknown. It is simple, requires only one number from the patient, and is the basis for almost every “due date calculator” online.

Conception-based dating

If you know the date of conception (for example from ovulation tracking or a single instance of intercourse), a separate formula applies:

Due date = conception date + 266 days (38 weeks)

The difference — 14 days — reflects the gap between the start of a cycle and ovulation. If your cycle is longer or shorter than 28 days, the LMP formula will systematically over- or under-estimate, which is why conception dating or an early ultrasound is sometimes preferred.

IVF transfer dating

With in-vitro fertilization, the embryo’s exact age at transfer is known, so neither LMP nor a “conception date” is needed. Instead:

• Day-5 blastocyst transfer: due date = transfer + 261 days
• Day-3 cleavage transfer: due date = transfer + 263 days

These formulas are calibrated so the resulting due date matches what a natural cycle with day-14 ovulation would have produced. Clinics still assign an “estimated LMP” for IVF pregnancies so the pregnancy can be compared with growth charts designed for natural cycles.

Why a due date is an estimate, not a guarantee

Only about 5% of babies are born on their exact due date. Normal full-term delivery happens anywhere from 37 to 42 weeks of gestation. A due date is simply the center of that bell curve — useful for timing scans, leave planning, and risk assessment, but not a prediction of the actual birth day.

Several factors push the real delivery date away from the estimate:

• Cycle length: cycles longer or shorter than 28 days shift ovulation and therefore shift the true conception date.
• Dating by ultrasound: a first-trimester scan between 8 and 14 weeks measures the baby’s size and is generally more accurate than a calendar calculation. Providers will sometimes revise the due date based on these measurements.
• Multiple pregnancies: twins and higher-order multiples typically deliver earlier than 40 weeks.
• Individual biological variation: even with perfect cycle data, labor onset varies.

The takeaway

Treat your due date as a useful anchor, not a fixed appointment. If you want a quick estimate, our pregnancy due date calculator supports all three methods above — LMP, conception, and IVF transfer — and our pregnancy week calculator will tell you exactly how many weeks and days pregnant you are today.

No online calculator can replace the dating scan and clinical judgment your provider brings. Always discuss your specific dates with your doctor or midwife.