The truth about due dates

Naegele's rule is a method used to estimate a woman's due date for delivery of a baby. It is named after Franz Karl Naegele, a German obstetrician, who first published the method in 1812.

According to Naegele's rule, the estimated due date can be calculated by adding 7 days to the first day of the woman's last menstrual period (LMP), and then subtracting 3 months. For example, if a woman's LMP was January 1, her due date would be October 8. This calculation assumes that a woman has a regular menstrual cycle of 28 days and ovulates on day 14 of her cycle.

Ultrasound measurements and other tests may be used to confirm or adjust the estimated due date as the pregnancy progresses. Early ultrasound measurements are typically the most accurate for determining gestational age and due dates based on Naegele’s rule.

However, and this is where things get interesting, the research actually suggests that Naegele's rule is not an accurate way to determine due dates. For about half of first time parents, they can expect spontaneous labor (labor starting by itself) to begin by 40 weeks, 5 days. This means that half of first time parents can expect their baby to arrive after 40 weeks, 5 days. Similarly, for people who have given birth before, the data shows that half can expect spontaneous labor by 40 weeks, 3 days. 

This means that for all birthing people, spontaneous labor will begin after their due date. So, to be more accurate, we should really be talking about due dates as a range of time, and not a specific date - less than 5%of babies are actually born on their due date!

So when the calls start rolling in from loving family and friends towards the very end of pregnancy to “check in” (checking to make sure you didn’t forget to tell them that you had your baby!) you can reassure them that the majority of babies actually come after their due dates, and that you won’t forget to share the exciting news when the time comes!

When was your due date, and when did your baby actually arrive? Share in the comments!