Pregnancy Due Date Calculator
Enter the first day of your last period and your usual cycle length. The result is an estimate, as most babies arrive within a few weeks of it.
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Formula
due date = first day of last period + 9 months + 7 days + (cycle length − 28) days
If the target month is shorter, the date is moved back to its last day before adding the 7 days. Trimester starts are 98 and 196 days after the last period began.
Worked example
Last period began 1 January 2024 with a 28-day cycle: plus 9 months is 1 October 2024, plus 7 days gives 8 October 2024. With a 32-day cycle add 4 more days for 12 October 2024.
Frequently asked questions
What is Naegele's rule?
Naegele's rule adds nine months and seven days to the first day of your last period. For 1 January 2024 that gives 8 October 2024.
How does cycle length change the due date?
The rule assumes a 28-day cycle. For each day your usual cycle is longer or shorter than 28, the date moves by one day, so a 32-day cycle moves 8 October to 12 October.
How accurate is an estimated due date?
Only about 4 in 100 babies are born on the estimated date. Most arrive between 37 and 42 weeks, so treat it as a guide.
When does each trimester begin?
The second trimester starts at week 14, which is 98 days after your last period began, and the third at week 28, 196 days after. From 1 January 2024 those are 8 April and 15 July.