A due date is an estimate: only about 1 in 20 babies arrives on it.
Source: Khambalia et al., 2013.
How the calculator gets to the date
The due date is the first day of your last period plus 280 days, or 40 weeks. That rule, Naegele's, assumes a 28-day cycle; when yours is shorter or longer, the calculator moves the date by the difference.
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The first day of your last period.
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Plus 280 days, or 40 weeks: Naegele's rule.
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The adjustment for your cycle: each day more or less than 28 days moves the date by one day. In a 32-day cycle, it moves 4 days later.
Pregnancy weeks are counted from the last period, not from conception.
Questions about the due date
How is the due date calculated?
With Naegele's rule: the first day of the last period plus 280 days (ACOG, 2017). The calculator adjusts that date when your cycle is not 28 days.
Why adjust for cycle length?
Because the rule assumes a 28-day cycle, and only 13% of cycles are exactly that long (Bull et al., 2019). In a longer cycle, ovulation comes later, and so does the date.
Will the baby arrive on the due date?
Rarely on the day itself: in a study of 18,708 births, only 5% happened on the due date (Khambalia et al., 2013). It is a reference point, not an appointment.
When do pregnancy weeks start counting?
From the last period, not from conception. So by the time of conception, a pregnancy is already counted as about two weeks along.
Sources
- American College of Obstetricians and Gynecologists. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstetrics & Gynecology, 2017;129(5):e150–4.
- Bull JR, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2019;2:83.
- Khambalia AZ, Roberts CL, Nguyen M, et al. Predicting date of birth and examining the best time to date a pregnancy. International Journal of Gynecology & Obstetrics, 2013;123(2):105–9.
General information. Not a substitute for your doctor. Updated September 27, 2026.