Health · 5 min read

How Is Your Due Date Calculated? Naegele's Rule Explained

Written by Muhammad Ajmal · Last reviewed 2026-08-15

Your estimated due date is not the date your doctor guesses you will go into labor. It is a calculated point based on a standard formula applied to your last period, and our due date calculator runs that same formula in seconds so you can see the date and check it against your own records.

The standard method: 280 days from your last period

Most due dates are calculated as 280 days, or 40 weeks, from the first day of your last menstrual period (LMP). This assumes a 28 day cycle with ovulation occurring around day 14, since counting from LMP is far easier to pin down than counting from the exact day of conception, which most people cannot state with certainty.

Naegele's rule explained

The formula behind this is called Naegele's rule, named after the German obstetrician who popularized it in the 1800s. To use it by hand: take the first day of your last period, subtract three calendar months, then add seven days and one year. A last period starting March 10 becomes December 10 after subtracting three months, then December 17 after adding seven days, then December 17 of the following year. The result lands close to the same date as simply adding 280 days.

Why counting starts before conception happened

This trips a lot of people up. Pregnancy is dated from the first day of your last period, which is roughly two weeks before ovulation and conception actually occur. So by the time a pregnancy test turns positive, you are already considered about four weeks along, even though fertilization happened only about two weeks earlier. It is a dating convention, not a claim about when the pregnancy biologically started.

When ultrasound dating takes over

An early ultrasound, usually done in the first trimester, measures the size of the embryo or fetus and can estimate gestational age more precisely than LMP alone, especially if your cycles are irregular or you are unsure of your last period date. If the ultrasound measurement differs from the LMP-based date by more than about a week, most providers will adjust your due date to match the ultrasound rather than the original calculation.

Irregular cycles change the math

Naegele's rule assumes a textbook 28 day cycle. If your cycles typically run longer or shorter than 28 days, ovulation happens later or earlier relative to your period, which shifts your real conception date without changing when your last period started. Some providers adjust the standard formula by adding or subtracting the difference between your average cycle length and 28 days.

Known conception date or IVF

If you know your conception date precisely, such as through IVF with a documented transfer date, the due date is calculated differently: 266 days from ovulation or fertilization, or a fixed number of days from a known embryo transfer date depending on whether it was a day 3 or day 5 transfer. This tends to be more accurate than an LMP-based estimate.

A date, not a deadline

The American College of Obstetricians and Gynecologists defines full term as 39 weeks 0 days through 40 weeks 6 days, with early term covering 37 weeks 0 days to 38 weeks 6 days and late term starting at 41 weeks. Your due date marks the middle of a normal delivery window, not a specific day your body is expected to hit. Enter your own last period date into the due date calculator to see your estimated date and how far along you are right now.

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