How to use the due date calculator
Choose whether you know the first day of your last period or the date of conception. If your cycle is usually longer or shorter than 28 days, change the cycle length and the date shifts to match.
The result is an estimate. A dating scan in the first trimester is the most reliable way to set a due date, and your midwife or doctor may change the date after it.
The formula
From the last period, the calculator uses Naegele's rule: due date = first day of last period + 280 days, adjusted by the difference between your cycle length and 28 days. From conception: due date = conception date + 266 days.
Worked example
If your last period started on 1 January and your cycle is 28 days, counting forward 280 days gives an estimated due date of 8 October.
This calculator gives a general estimate for adults. It is not medical advice. Talk to a doctor or other qualified health professional about your own situation.
Common questions
How likely is my baby to arrive on the due date?
Only around 1 in 20 babies do. Most arrive in the two weeks either side.
Why is pregnancy counted from the last period?
The date of conception is rarely known for certain, while the start of the last period usually is. That is why 40 weeks of pregnancy include about two weeks before conception.
What if my cycles are irregular?
The estimate will be less reliable. An early ultrasound scan gives a better date.