How a Pregnancy Due Date Is Calculated
A due date is usually 280 days from the first day of your last period. It is an estimate — very few babies arrive on exactly that day.
The first question after a positive test is always when the baby will arrive. The answer is an estimate, called the due date.
The calculation
The common method adds 280 days — 40 weeks — to the first day of your last period. It is called Naegele’s rule, and it assumes a 28-day cycle.
Check your estimated due date→If your cycle is not 28 days
Many women have longer or shorter cycles. The difference is added to or taken off the calculation — a 32-day cycle pushes the date about four days later. Our calculator adjusts for this.
Why pregnancy is counted from a period
It seems odd, and the reason is practical: the date of conception is usually unknown, while the date of the last period is remembered. Counting starts there — which means the first two weeks technically precede the pregnancy itself.
Why an ultrasound is more accurate
If your periods are irregular, or you do not remember the last date, the calculation is unreliable. An early ultrasound measures the baby and sets a date from that, which is considered more accurate. Your doctor works from that date.
How many babies arrive on the day?
Very few — around 4 per cent. Most births happen in the weeks around the due date. Treat it as a window rather than a fixed date; knowing that takes a lot of the anxiety out of it, especially once the date passes.
Frequently asked questions
Is the due date certain?+
No, it is an estimate. Only about 4% of babies arrive on exactly that day.
What if my periods are irregular?+
The calculation is then unreliable. An early ultrasound gives a better date.
How long is a pregnancy?+
Usually 40 weeks, counted from the first day of the last period.
What if the date passes?+
That is not unusual. What happens next is decided by your doctor based on your checks.