Pregnancy due date calculator
Enter the first day of your last menstrual period and your usual cycle length. The calculator gives your estimated due date, how far along you are today and your trimester. An early ultrasound scan gives a more exact date.
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How it's calculated
Naegele's rule: the due date is 280 days (40 weeks) after the first day of the last period, adjusted by the difference between your cycle length and 28 days. Only about 1 in 20 babies arrives on the due date; most arrive within two weeks either side.
Trimesters
| Trimester | Weeks |
|---|---|
| First | Up to 13 weeks 6 days |
| Second | 14 to 27 weeks 6 days |
| Third | 28 weeks to birth |
Frequently asked questions
How accurate is a due date calculator?
It is an estimate. A dating ultrasound in the first trimester is more accurate, especially with irregular cycles.
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