Enter the first day of your last period to estimate a due date based on a 40-week pregnancy.
How due dates are estimated
A pregnancy due date is most commonly estimated using Naegele rule, which adds 280 days, or forty weeks, to the first day of the last menstrual period. It assumes a regular twenty eight day cycle, which is why it is an estimate rather than a fixed appointment.
In practice only a small fraction of births happen on the predicted day, with most arriving in the two weeks on either side. For people with irregular cycles, an early ultrasound dating scan gives a more reliable estimate than the calendar method.
This tool is for general information only and is not medical advice. Your healthcare provider gives the authoritative due date and adjusts it as the pregnancy progresses.
The formula
Worked example
An LMP of March 1 gives an estimated due date about 280 days later, in early December.
How to read your result
This is an estimate using Naegele's rule and assumes a regular 28-day cycle. It is not medical advice; your provider gives the authoritative date.
Frequently asked questions
- How accurate is this?
- It is a standard estimate; only about 1 in 20 births occur on the exact due date.
- What if my cycle is irregular?
- The estimate is less reliable; an ultrasound dating scan is more accurate.
- Is this medical advice?
- No. Always confirm with a healthcare professional.
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