How is a due date calculated?

The common method adds 280 days (40 weeks) to the first day of your last period. It assumes a regular 28-day cycle, so a scan is used to confirm or adjust the date.

When are twins usually due?

Twins share the same estimated due date as a single baby, but they very often arrive earlier, frequently around 36 to 37 weeks, so it is wise to be ready before the date.

How accurate is this?

It is a reasonable estimate for a regular cycle. A dating scan in early pregnancy is more accurate, and your midwife will give you the date to go by.

Due date calculator: when is my baby due?

Due date calculator

First day of your last period

An estimate, worked out in your browser. Nothing you enter is sent anywhere.

How the estimate works

The calculator counts 40 weeks from the first day of your last period. This is the standard starting point midwives use, and a dating scan in the first trimester then confirms or adjusts it.

If you are expecting twins

Twins are given the same estimated due date, but they commonly arrive several weeks earlier than a single baby. It is worth having the essentials, and a plan for tracking two babies, ready before the date rather than on it.

Quick answer

This due date calculator estimates your due date as 40 weeks (280 days) from the first day of your last period, and shows roughly how many weeks pregnant you are today. It runs in your browser. It is an estimate: a dating scan is more accurate, and twins often arrive earlier, frequently around 36 to 37 weeks.

How to use it

Enter your last period. Set the first day of your last menstrual period.
Read your due date. You get an estimated due date, 40 weeks on, and your current stage of pregnancy.
Confirm with your midwife. A dating scan gives a more accurate date; use this as a quick estimate in the meantime.

Frequently asked questions

How is a due date calculated?: The common method adds 280 days (40 weeks) to the first day of your last period. It assumes a regular 28-day cycle, so a scan is used to confirm or adjust the date.
When are twins usually due?: Twins share the same estimated due date as a single baby, but they very often arrive earlier, frequently around 36 to 37 weeks, so it is wise to be ready before the date.
How accurate is this?: It is a reasonable estimate for a regular cycle. A dating scan in early pregnancy is more accurate, and your midwife will give you the date to go by.