How is a pregnancy due date calculated?

A pregnancy due date — also called the estimated due date (EDD) or estimated date of delivery — is a target date for when a baby is expected to be born. Almost every method for calculating it is built on the same idea: pregnancy lasts about 40 weeks (280 days) from the first day of the last menstrual period.

But behind that simple sentence are several different methods, assumptions, and adjustments. This guide walks through each one so you understand what your due date means and why a doctor, a phone app, and an online calculator might disagree by a day or two.

Method 1: Naegele's rule (LMP-based)

Naegele's rule is the oldest and most widely-used method. Developed in the early 1800s, it works in three steps:

1. Take the first day of your last menstrual period (LMP).
2. Add one year.
3. Subtract three months and add seven days.

The result is your estimated due date. Mathematically, this is the same as adding 280 days to the LMP. Most online due-date calculators (including ours) use exactly this formula.

Method 2: Adjusted for cycle length

Naegele's rule assumes a 28-day cycle. If your cycle is longer or shorter, ovulation happens earlier or later, and the due date shifts accordingly. The adjustment is straightforward:

EDD = LMP + 280 days + (cycle length − 28)

So a 30-day cycle pushes the due date forward by 2 days. A 26-day cycle pulls it back by 2 days. Our due date calculator handles this adjustment automatically when you set your cycle length.

Method 3: Conception date

If you know the exact day of conception (often the case after a single fertile encounter or with ovulation tracking), you can count forward instead. Pregnancy is 266 days from conception:

EDD = conception date + 266 days

Method 4: Ultrasound dating

A first-trimester ultrasound (typically between 7 and 13 weeks) measures the crown-rump length (CRL) of the fetus. The CRL gives a gestational age accurate to within ± 5–7 days, which is more accurate than LMP dating for women with irregular cycles or unknown LMP.

ACOG (the American College of Obstetricians and Gynecologists) recommends that if first-trimester ultrasound dating differs from LMP dating by more than seven days, the ultrasound dating should be used to set the due date.

Method 5: IVF and ART cycles

For pregnancies achieved through in-vitro fertilisation (IVF) or other assisted reproductive technology, the embryo age and transfer date are known precisely. This makes IVF dating more accurate than LMP dating. The general formula is:

• Day 3 transfer: EDD = transfer date + 263 days
• Day 5 transfer: EDD = transfer date + 261 days
• Day 6 transfer: EDD = transfer date + 260 days

See our IVF due date calculator for details on fresh vs frozen (FET) cycles.

What if my dates do not match?

It is common for the LMP-based date and the ultrasound-based date to differ by a few days. Doctors usually follow this hierarchy:

1. If a first-trimester ultrasound is available, it takes precedence over LMP dating when the difference is greater than seven days.
2. For IVF pregnancies, the IVF-derived date is used regardless of LMP or later ultrasounds.
3. Late-pregnancy ultrasounds (after 22 weeks) are not used to revise the due date — they are too imprecise.

How accurate is the due date?

Only about 5% of babies are born on their estimated due date. Most deliveries (about 80%) happen between 37 and 42 weeks. The due date is best understood as the centre of a probability distribution, not a deadline.

Pregnancies that go past 42 weeks are called "post-term" and most providers will discuss induction options around 41 weeks. Pregnancies that deliver before 37 weeks are considered preterm.

Try the calculators

• Due Date Calculator — supports LMP, conception, ultrasound, and known due dates.
• IVF Due Date Calculator — for Day 3, Day 5, Day 6 transfers and FET cycles.