How a due date is worked out
Doctors count a pregnancy from the first day of your last period, not from conception. A due date is 280 days (40 weeks) after that day, for a 28-day cycle. If your cycle is longer or shorter, ovulation usually comes later or earlier. So the calculator moves the due date by the same number of days. That’s why it asks for your cycle length. This adjustment is an estimate, not part of ACOG’s rule, and an early ultrasound gives a firmer date.
If you know the conception date, the due date is 266 days after it. If you conceived through IVF, use the IVF due date calculator, which counts from your embryo’s age.
Why your ultrasound date might differ
An ultrasound in the first trimester (up to 13 weeks and 6 days) is the most accurate way to date a pregnancy that didn’t come from IVF. The American College of Obstetricians and Gynecologists (ACOG) says to switch to the ultrasound date if it differs from your period-based date by more than 7 days. Before 9 weeks, a difference of more than 5 days is enough. Later scans have wider limits.
How often babies arrive on the due date
Not often. In an Australian study of nearly 19,000 births where labor started on its own, only 5% happened on the due date, about 1 in 20. About two in three came within a week of it.
Sources
- American College of Obstetricians and Gynecologists, Committee Opinion 700, “Methods for Estimating the Due Date” (2017). acog.org
- American College of Obstetricians and Gynecologists, Committee Opinion 579, “Definition of Term Pregnancy.” acog.org
- CDC, “Tdap vaccination during pregnancy” and “RSV vaccine for pregnant people.” cdc.gov, cdc.gov
- Khambalia AZ, et al. “Predicting date of birth and examining the best time to date a pregnancy.” International Journal of Gynaecology and Obstetrics, 2013. doi.org