Find your estimated due date
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How the due date is worked out
ACOG says the 280-day rule assumes a regular 28-day cycle with ovulation on day 14, so it does not account for irregular cycles, variable ovulation or trouble remembering the date.[1]
ACOG says pregnancy is counted from the first day of the last period, so 2 weeks are counted at the beginning when you are not yet pregnant.[2] The UK's NHS also dates pregnancy from the first day of your last period.[3]
For pregnancies from IVF, ACOG says the due date is set from the embryo's age and the transfer date: 261 days after the transfer of a day-5 embryo, or 263 days after a day-3 embryo.[1]
An early ultrasound may change the date
ACOG says an ultrasound in the first trimester, up to 13 weeks and 6 days, is the most accurate way to set or confirm how far along a pregnancy is.[1]
It says that if an ultrasound before 14 weeks differs from the date based on the last period by more than 7 days, the due date is changed to match the ultrasound.[1] Before 9 weeks, it says a difference of more than 5 days is enough.[1]
NICHD says working out the due date is part of the first prenatal visit.[4] MedlinePlus says a first-trimester ultrasound helps estimate the due date.[5]
Will the baby come on that day?
ACOG says only about 1 in 20 women give birth on their due date.[2]
The NHS says most women go into labor within a week either side of their due date.[6]
MedlinePlus says labor usually starts between 37 and 42 weeks.[7]
Questions families ask
How is a due date calculated?
ACOG says that, by convention, the due date is 280 days after the first day of the last menstrual period.[1]
Can my due date change?
ACOG says a first-trimester ultrasound may change the due date if it differs from the date based on the last period by more than 7 days, or more than 5 days before 9 weeks.[1] It says changes should be rare and discussed with the patient.[1]
How do you calculate a due date after IVF?
ACOG says it is 261 days after the transfer of a day-5 embryo, or 263 days after a day-3 embryo.[1]
How many babies are born on their due date?
ACOG says only about 1 in 20 women give birth on their due date.[2]
Free from UpOnly Learning. Written for families of every faith. No outside ads and no affiliate links. Every fact on this page comes from the sources listed below. How we source and review these pages
Updated 2026-10-24: first published.
Sources
All sources were opened and checked on the date shown.
- American College of Obstetricians and Gynecologists (with AIUM and SMFM). Methods for Estimating the Due Date (Committee Opinion No. 700; reaffirmed 2025). Checked October 24, 2026.
- American College of Obstetricians and Gynecologists. How Your Fetus Grows During Pregnancy. Checked October 24, 2026.
- NHS (United Kingdom). 4 weeks pregnant guide. Checked October 24, 2026.
- Eunice Kennedy Shriver National Institute of Child Health and Human Development (NIH). What happens during prenatal visits?. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Prenatal care in your first trimester. Checked October 24, 2026.
- NHS (United Kingdom). 39 weeks pregnant guide. Checked October 24, 2026.
- MedlinePlus (National Library of Medicine). Childbirth. Checked October 24, 2026.