Due date

How your due date is calculated, and why it can change

By Nine Moons EditorialMedically reviewed by Professor Dr FarhatUpdated 9 Oct 20265 min read

Quick answer

Your due date is usually 280 days, or 40 weeks, after the first day of your last period. It's adjusted if your cycle is longer or shorter than 28 days. Your dating scan between 8 and 14 weeks is more accurate, so your doctor may change the date after it. Only around 1 in 20 babies arrive on the day itself.

Key facts

From last period	First day of last period + 280 days
From conception	Conception date + 266 days
IVF day-5 transfer	Transfer date + 261 days
IVF day-3 transfer	Transfer date + 263 days
Full term	39 weeks 0 days to 40 weeks 6 days

The standard method: Naegele's rule

Doctors count from the first day of your last menstrual period (LMP), not from conception. Add one year, take away three months, and add seven days. For example, an LMP of 10 January gives a due date of 17 October.

This assumes a 28-day cycle. If your cycle is 32 days, ovulation happens about 4 days later, so add 4 days. If it's 25 days, take 3 off. Our due date calculator does this for you.

Why the scan may change your date

Measuring the baby between 8 and 14 weeks is the most accurate way to date a pregnancy. If the scan date differs from your period date by more than about 5 to 7 days, your doctor will usually go with the scan. After that, the date shouldn't change again, even if later scans show a bigger or smaller baby.

IVF due dates

With IVF, the exact age of the embryo is known, so these dates are the most precise. Use the IVF option in the calculator with your transfer date and embryo age.

What "term" really means

Name	Weeks
Preterm	Before 37 weeks
Early term	37+0 to 38+6
Full term	39+0 to 40+6
Late term	41+0 to 41+6
Post-term	42+0 and beyond

Most babies arrive between 37 and 42 weeks. Many hospitals offer induction between 41 and 42 weeks.

Follow your pregnancy week by week

Once you know your date, open your week-by-week guide to see your baby's size, what your body is doing and what your partner can do to help.

Frequently asked questions

Why do they count from my period if I conceived 2 weeks later?

Because most women know their period date but not their conception date. The first 2 weeks are counted as part of pregnancy.

Can my due date change?

Usually only once, after the dating scan. Later growth scans don't change it.

Are first babies usually late?

First babies are slightly more likely to arrive after the due date, but the difference is small.

Sources

ACOG Committee Opinion 700: Methods for estimating the due date
NICE NG201 Antenatal care
NHS: Your due date

General information, not medical advice. Medical disclaimer · How we review content

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