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

Q: What is Naegele's rule?

Take the first day of your last menstrual period, add one year, subtract three months, then add seven days. It is a calendar shortcut for adding 280 days — 40 weeks — and has been the standard method since the early 1800s.

Q: How many days is a full pregnancy?

280 days from the first day of the last menstrual period, which is about 266 days from conception. The 14-day difference is the gap between the period starting and ovulation occurring in a textbook 28-day cycle.

Q: How is a due date calculated for IVF?

From the transfer date, because the embryo's age is known exactly. A day-5 blastocyst transfer gives a due date 261 days later; a day-3 transfer gives 263 days. IVF due dates are the most precise there are, since no assumption about ovulation is needed.

Q: What if my cycle is not 28 days?

Naegele's rule assumes ovulation on day 14. If you ovulate later — common with a 35-day cycle — the true due date is later than the formula suggests, by roughly the difference between your ovulation day and day 14. An early ultrasound resolves this properly.

Q: How accurate are due dates?

Fewer than 1 in 20 babies are born on their exact due date. Most arrive within a two-week window either side, which is why 37 to 42 weeks is considered a normal range rather than a single target date.

Naegele's rule

LMP + 1 year − 3 months + 7 days
= LMP + 280 days (40 weeks)

Worked through with an LMP of 10 January 2026:

Step	Date
Last menstrual period	10 Jan 2026
Add one year	10 Jan 2027
Subtract three months	10 Oct 2026
Add seven days	17 Oct 2026
Estimated due date	17 Oct 2026

Adding 280 days directly to 10 January gives the same answer. The year-and-months version simply existed before anyone wanted to count 280 days by hand.

From LMP, conception day or IVF transfer.Due date calculator →

The three starting points

Known date	Add	Why
Last menstrual period	280 days	The standard method
Conception / ovulation	266 days	Skips the pre-ovulation fortnight
IVF day-5 transfer	261 days	Embryo already 5 days old
IVF day-3 transfer	263 days	Embryo already 3 days old

IVF dates are the most reliable of all. Every other method has to guess when ovulation happened. With a transfer date, the embryo's exact age is known, so there is nothing left to estimate.

The assumption hidden in the formula

Naegele's rule assumes a 28-day cycle with ovulation on day 14. Real cycles range from about 21 to 35 days, and the variation sits almost entirely in the first half — the phase before ovulation.

Cycle length	Likely ovulation	Adjustment to EDD
24 days	Day 10	4 days earlier
28 days	Day 14	none
32 days	Day 18	4 days later
35 days	Day 21	7 days later

A week is a meaningful difference near the end of pregnancy, when decisions about induction depend on gestational age — which is why an early scan matters more than the formula.

Why ultrasound takes over

A first-trimester scan measures the embryo directly, and at that stage growth is remarkably consistent between pregnancies. Accuracy is about ±5 to 7 days. If a scan disagrees with the LMP date by more than that margin, clinicians normally redate the pregnancy to the scan.

Accuracy declines as pregnancy progresses — by the third trimester, size reflects genetics and growth conditions as much as age, so a late scan is a poor dating tool. This is why the earliest scan is the one that sets the date, and later ones do not move it.

A date, not a deadline

When babies actually arrive

<37w preterm 37–39w early term 39–41w full/late term 41w+ late & post-term

Fewer than one baby in twenty arrives on the estimated date itself. Most come within two weeks either side, which is precisely why 37 to 42 weeks is treated as normal rather than a single target.

This explains the arithmetic behind due dates. It is general information, not medical advice — your clinician's dating and guidance take precedence, and anything that concerns you about your pregnancy should go to them.

Run your own numbers

Due Date Pregnancy Conception

FAQ

What is Naegele's rule?

Take the first day of your last menstrual period, add one year, subtract three months, then add seven days. It is a calendar shortcut for adding 280 days — 40 weeks — and has been the standard method since the early 1800s.

How many days is a full pregnancy?

280 days from the first day of the last menstrual period, which is about 266 days from conception. The 14-day difference is the gap between the period starting and ovulation occurring in a textbook 28-day cycle.

How is a due date calculated for IVF?

From the transfer date, because the embryo's age is known exactly. A day-5 blastocyst transfer gives a due date 261 days later; a day-3 transfer gives 263 days. IVF due dates are the most precise there are, since no assumption about ovulation is needed.

What if my cycle is not 28 days?

Naegele's rule assumes ovulation on day 14. If you ovulate later — common with a 35-day cycle — the true due date is later than the formula suggests, by roughly the difference between your ovulation day and day 14. An early ultrasound resolves this properly.

How accurate are due dates?

Fewer than 1 in 20 babies are born on their exact due date. Most arrive within a two-week window either side, which is why 37 to 42 weeks is considered a normal range rather than a single target date.

Sources

Primary references used for the figures and rules on this page.

Methods for Estimating the Due Date — ACOG
How Your Fetus Grows During Pregnancy — ACOG