Due date calculator
This due date calculator estimates when your baby is due from the first day of your last period, a known conception date, an ultrasound scan or an IVF embryo transfer. It gives the likely range around the date as well as the date itself.
A due date is an estimate
Only about 5 in 100 babies arrive on the day itself, and about 66 in 100 within a week of it (Khambalia and colleagues, 2013, in 18,708 single births after spontaneous labour in one Australian health district). That is why the result comes with a range. Your midwife or doctor may change the date after a dating scan — the calculator shows you when, and why, that happens.
The due date is 280 days after the first day of your last period
By convention the due date is 280 days after the first day of the last menstrual period. That count assumes a regular 28-day cycle with ovulation on day 14, so it makes no allowance for a different cycle length or for a period date that is remembered wrongly. ACOG reports that only about half of women recall the date of their last period accurately (ACOG Committee Opinion 700, 2017, reaffirmed 2022).
An early ultrasound scan can change the date
In the US, ACOG describes an ultrasound measurement in the first trimester, up to and including 13 weeks and 6 days, as the most accurate way to establish or confirm gestational age. If a scan before 14 weeks differs from the last-period date by more than 7 days, ACOG advises changing the due date to the scan date; before 9 weeks the threshold is more than 5 days. After IVF the due date is assigned from the age of the embryo and the date of transfer (ACOG Committee Opinion 700). In the UK, NICE recommends offering a scan between 11 weeks and 2 days and 14 weeks and 1 day to determine gestational age (NICE NG201, 2021).
Sources
- American College of Obstetricians and Gynecologists, American Institute of Ultrasound in Medicine and Society for Maternal-Fetal Medicine. Methods for estimating the due date (Committee Opinion No. 700). Obstetrics & Gynecology. 2017;129(5):e150–e154. doi:10.1097/AOG.0000000000002046. Reaffirmed 2022.
- Khambalia AZ, Roberts CL, Nguyen M, Algert CS, Nicholl MC, Morris J. Predicting date of birth and examining the best time to date a pregnancy. International Journal of Gynecology & Obstetrics. 2013;123(2):105–109. doi:10.1016/j.ijgo.2013.05.007. PMID 23932061.
- National Institute for Health and Care Excellence. Antenatal care (NG201). Published 19 August 2021; last reviewed 27 December 2024.