• Sarah Maria

The Truth about your Due Date


Or why the due date should be called Estimated Timerange of Arrival


Ever wondered how the due date was invented and how the calculation works? Or even more interesting: is the due date reliable?


Before I want to dive into the history and research of the due date I would like to explain the lengths of pregnancy.


Midwives, obstetricians and doctors define the late weeks of pregnancy based on the maturity of the fetus. Information like the survival rate, the maturity of the lungs or the likelihood of contractions contributes to this classification.


Babies born

  • before 37 weeks are considered Preterm

  • between 37 weeks + 0 days and 38 weeks +6 days are considered Early Term

  • between 39 weeks +0 days and 40 weeks +6 days are labelled Full Term

  • between 41 weeks +0 days and 41 weeks +6 days are called Late-term

  • at 42 weeks +0 days or later are considered Post Term

As you can see Full Term does not describe one exact day, rather it includes a range of two full weeks. But who invented the due date?


During medieval times upper-class, royal or noble pregnant women withdraw from the society around the estimated date of birth. This was called Confinement and people believed this would calm the mother in order to reduce preterm birth and stress. Only Midwives, female family members and the doctors were allowed to visit.

Quite astonishing if we think about pregnant women today who sometimes even have to work until their due date or worse until contractions start.


The Calculation of the gestational length goes back to 1806. Dr Franz Karl Naegele, a German obstetrician devised the rule that is still used today. Naegele became a professor and director of a hospital in Heidelberg and also wrote a textbook for midwives.


Naegele created a rule for calculating childbirth with a gestational age of 280 days (aka 40 weeks). The calculation works as follows:


Take the day of your last menstrual period (LMP)

  • add 1 year

  • subtract 3 months

  • add 7 days

For example, let us assume the 10th of April 2020 was your last period

  • add 1 year: 10th of April 2020

  • - 3 months: 10th of January 2021

  • +7 days: 17th of January 2021 --> your due date if you use Naegele's rule


The only obstacle while calculating with this easy method is, that Naegele assumes every woman has a cycle of 28 days. AND as you can read above, I only mentioned using the day of your last period. Before Naegele, there was Hermann Boerhaave, an obstetrician from the Netherlands. While studying 100 pregnant women in 1744 he found out, that by adding 7 days and nine months to the last period, you can calculate the due date. But both doctors never defined "day of your last period". Is it the first day of your last period or the last day?


Before 1900 most doctors calculated by applying the rule using the last day of the last period (the last day you bleed) And suddenly, as the century changed, so did medical textbooks: from 1900 on most doctors and health professionals used the first day of the last period, until today.


So how are we really sure which date to choose if it has no real scientific evidence?


Thank god for ultrasounds!


The ultrasound found its way into medicine in the 1970s. And soon after, obstetricians started to define gestational age by measuring the baby via ultrasound. Large reviews and studies indicate that using the ultrasound to define the gestational age is more accurate than calculating with the LMP. Women, who used LMP to mark the due date were more likely to be labelled post-term and therefore, to be induced.


This is most likely because every woman has a different cycle length and you can never be certain of the exact day when the embryo nests into the uterine wall or the exact day of ovulation.


According to different authors, the best time to define the gestational age via ultrasound seems to be between 11 to 14 weeks. Everything later than 20 weeks also leads to higher uncertainties aka more post-term births.


To Come back to the Estimated Date of Delivery: By looking into history and diversities in human bodies it seems clear that calculating a certain day for giving birth does not seem accurate or even reasonable. Unfortunately there is hardly any research regarding the numbers of pregnant women giving birth at certain weeks of gestation because most doctors start to induce around 40 to 41 weeks. So there is no data available to compare. Women are being induced earlier to make sure the baby stays healthy, because sadly we know the longer we wait the higher risks we have


Caughey et al., (2003) and Caughey & Musci, (2004) found a higher risk for

  • infection of the membranes / or the uterus

  • placenta malfunctions or abruption

  • preeclampsia

  • Cesarean section

  • Neonatal intensive care after birth

  • low 5 minute Apgar or even stillbirth

... the more advanced the pregnancy gets (meaning more than 42 weeks).


The few studies we have show that it is most likely for first time mothers to give birth between 39 weeks + 2 days and 40 weeks +5 days. So next time your OB-GYN or midwife tells you your exact due date, remember thinking of it as a range of time where it is possible you give birth, always count +/- 5 to 6 days.


Because as you remember in midieval times it was considered stressful for the pregnant women to be in society during due time, so to avoid you and / or your family worrying, waiting and impatiently wishing for birth, enjoy the last days of pregnancy you have, it could be the last time of your life being pregnant. Enjoy while you can. And the stress and sleeplesness is propably nothing compared to parenthood ;)



Stay safe, stay healthy and remember


it's not pain, but power


your midwife Sarah




Photo by Tim Bish on Unsplash









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