About the Pregnancy Due Date Calculator
The standard estimate, Naegele's rule, counts 280 days from the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14, so this calculator lets you enter your own cycle length and adjusts accordingly — a meaningful difference for anyone whose cycles run long or short.
How to use the Pregnancy Due Date Calculator
- Enter the first day of your last menstrual period.
- Adjust the average cycle length if it is not 28 days.
- Read your estimated due date and how far along you are today.
- Check the trimester end dates for appointment planning.
Why use this tool?
Cycle length adjusted
A 32-day cycle shifts the estimate later by four days — the calculator applies that automatically.
Weeks and days
Shows gestational age the way clinicians write it, as weeks plus days.
Trimester milestones
End dates for the first, second and third trimesters for scans and leave planning.
Frequently asked questions
How accurate is a due date?
It is an estimate. Only about 4% of babies arrive exactly on the date; most arrive within two weeks either side, and a first-trimester dating ultrasound is more accurate than the last-period method.
Why count from the last period rather than conception?
Because the date of the last period is usually known and the date of conception rarely is. It means the first two weeks of a 'pregnancy' precede conception.
Can I use this after an IVF transfer?
No — IVF dating uses the transfer date and embryo age instead. Your clinic will provide the correct due date.