Let’s talk about due dates! Let’s be honest- your baby doesn’t have a crystal ball. You don’t magically hit 40 weeks and go into labour.
There are so many variations on how due dates are estimated, and what a ‘normal’ gestational age is, which to me, goes to show how much ‘normal’ range there is in terms of actual birth dates.
So how are due dates measured?
Centuries ago, a professor named Boerhaave looked at the records of 100 women and figured an estimated due date by adding 7 days to the last period, and then add 9 calendar months. Later, a professor named Naegele adapted this thinking and it was dubbed “Naegele’s rule”. This rule automatically assumes you have a 28 day cycle, and ovulate on day 14. Neither professor specified when the 7 days should be added - to the first day of the last period, or to the last day of the last period?
By the 1900s American textbooks had adapted the rule that the 7 days were added to the first day of the last period, a rule that is based on no real evidence, and may have not been an intention. This is the rule most health care providers go by today. Another way to calculate this is by adding 40 weeks onto the first day of your last period. But what about the people who do not ovulate on day 14? In the 1970’s ultrasound started being used as a measurement of the pregnancy, and found that if you go by the ultrasound measurement, people were less likely to be determined as “post term” (pregnant over 42 weeks). Why? This ultrasound measurement took into consideration people who had irregular menstrual cycles, did not ovulate on day 14, weren’t certain of their last period start date, or an embryo who took longer to implant. The most accurate time to have a dating ultrasound is between 8-16 weeks, with the accuracy noticeably declining after 20 weeks gestation. So what is a normal gestation range? 2 studies have shown that the AVERAGE for a person to birth is about 5 days after their ‘due date’. The first study was completed with 1514 healthy birthing people who’s due date (as calculated by Naegele’s Rule) perfectly matched with a first trimester ultrasound due date. Statistics showed that a first time birther gave birth on average 5 days after their due date, and 3 days after their due date for those who had given birth before.
The second study was a smaller final group of 113 people, but they were followed months before conception with hormone tracking, through to delivery. This means researchers new exactly when participants ovulated, conceived, and when implantation occurred. The research from this study showed that the median time frame for delivery was 40 weeks and 5 days from the first day of their last period. One person from this trial gave birth at 45 weeks and 6 days (from the first day of their last period) - which sounds SHOCKING! However, from the actual date of ovulation, gestational age was only 40 weeks and 4 days. From this study they also found that people who had a late rise in progesterone around conception also had a 12 days shorter pregnancy on average. So what does this mean?! It means there is a varying range of normal, and so many things to take into account. Don’t feel discouraged if you go beyond that 40 week mark. Induction isn’t a bad thing, but don’t let health care providers pressure you into induction just because you hit that magic number. If safe to do so, and you’re content, your baby is perfectly capable of choosing their own birthday -Mykayla xo