03/10/2026
How do we determine your due date?
Generally, your due date is 40 weeks after your last period - which means yes, it includes about 2 weeks where you weren’t pregnant at all! This method was popularized by Franz Naegele, an obstetrician from the 1800’s.
If you’re doing the math in your head you would usually add a year to the date of your last period, subtract 3 months, then add 7 days. This method assumes a 28 day menstrual cycle, and we’ve since developed a few different ways to adjust if that’s not the case.
The gold standard for accurate due dates uses estimation of growth from a first trimester ultrasound. I often recommend these so that we don’t feel unnecessary pressure from inaccurate dating. Research tells us that people who have an early ultrasound to date their pregnancy are less likely to be induced for a post-term pregnancy.
Evidence Based Birth (if you haven’t visited this site go and check it out!!) has an excellent article summarizing the information we have on due date accuracy. It states that research shows 50% of all women having their first baby will birth by 40wks 5 days, while 75% birth by 41wks and 2 days.
Several factors influence the date of birth in a normal healthy pregnancy such as menstrual cycle length, ovulation date, how long it takes the embryo to implant after fertilization, unusual hormonal reactions to early pregnancy (such as a late rise in progesterone), etc. We also have to consider family history, as it’s a strong predictor of how long your pregnancy might last!
Want to know more? Check out the article in the comments!
