Question 1

How is my due date calculated?

Accepted Answer

This calculator uses Naegele's rule: it adds 280 days (40 weeks) to the first day of your last menstrual period, adjusted for your cycle length if it is not 28 days. Only about 1 in 20 babies arrive exactly on the due date — most come within two weeks either side.

Question 2

What if my cycle is not 28 days?

Accepted Answer

Enter your average cycle length and the calculator adjusts the due date accordingly — a longer cycle pushes the date later, a shorter one earlier. This is because ovulation, and therefore conception, happens later or earlier in your cycle.

Question 3

What is gestational age?

Accepted Answer

Gestational age is how far along the pregnancy is, counted in weeks and days from the first day of your last period — not from conception. The calculator shows your current gestational age and which trimester you are in.

Question 4

When does each trimester start?

Accepted Answer

The first trimester runs from week 1 to the end of week 12, the second from week 13 to the end of week 27, and the third from week 28 until birth. The calculator tells you which one you are currently in.

Question 5

Is this a substitute for medical advice?

Accepted Answer

No. The estimate is a useful guide, but your midwife or doctor will confirm your due date, often with an early ultrasound, which can be more accurate than the last-period method. Always follow professional medical advice during pregnancy.

Due Date Calculator

How to use the due date calculator

How the due date is worked out

Frequently asked questions