About the Due Date Calculator
An estimated due date is calculated from the first day of the last menstrual period using Naegele’s rule, which assumes a 28-day cycle. This calculator adjusts for a different cycle length and also shows current gestational age and the dates each trimester begins.
How to use the Due Date Calculator
- Enter the first day of your last menstrual period, or a known conception date.
- Adjust the average cycle length if yours is not 28 days.
- Read the estimated due date and how many weeks along the pregnancy currently is.
- Check the trimester dates for planning appointments.
The formula behind it
Naegele’s rule: due date = first day of last period + 280 days, adjusted by (cycle length − 28) days.
Frequently asked questions
How accurate is an estimated due date?
Only about 4% of babies arrive on the exact date. Roughly 90% are born within two weeks either side, which is why clinicians treat it as the centre of a window rather than a deadline.
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 "pregnancy" precede conception, which is a known quirk of the convention.
Does cycle length matter?
Yes. Ovulation timing shifts with cycle length, so a 35-day cycle moves the due date about a week later than the standard 28-day assumption.
Is an ultrasound date more reliable?
Generally yes, especially a first-trimester scan, and clinicians will revise the due date if the scan differs materially from the period-based estimate.
This is an estimate only and is not a substitute for antenatal care. Always follow the dating given by your midwife, obstetrician, or ultrasound scan.