Your dates
Naegele's rule, adjusted for cycle length. An ultrasound date is more accurate. Not medical advice.
Forty weeks from the last period, not from conception
Pregnancy is dated from the first day of the last menstrual period, which is roughly two weeks before conception actually happens. That convention makes a pregnancy forty weeks rather than thirty-eight, and it is why you are counted as two weeks pregnant before there is anything to be pregnant with.
The due date is a midpoint, not an appointment
Fewer than one birth in twenty happens on the estimated date. Most arrive within two weeks either side, and a first baby tends to be late. Treating the date as a centre of a range rather than a deadline saves a lot of anxiety in the last month.
A scan beats the arithmetic
This calculation assumes a 28-day cycle with ovulation on day 14. Early ultrasound dating measures the pregnancy itself and is more accurate, so if a scan has given you a date, that is the one to use.
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Frequently asked questions
Q. What if my cycle is not 28 days?
Adjust the cycle length field and the due date shifts with it. A 35-day cycle pushes ovulation about a week later, and the due date with it.
Q. What are the trimesters?
Weeks 1–13, 14–27 and 28 onwards, though the exact boundaries vary between sources by a week or so. They are a convenience for describing the pregnancy, not clinical thresholds.
Q. How is the due date calculated?
By Naegele's rule: last period plus 280 days, adjusted here for cycles other than 28 days. It has been in use since the nineteenth century and remains the standard starting point.