Them: “OH MY GOODNESS! Look at you!!! How far along are you? When are you DUE?!?!?! How are you feeling?”
You, in the few weeks of pregnancy: “When is this baby ever going to come out?!?!”
Ah, yes. The endless questions and unknowns of the prenatal journey, specifically the end of the third trimester. If feels like all we are doing is waiting on the birth of a baby. Will the baby be early? Late? On time? What does on time even mean? Isn’t my due date accurate? How do we even know for sure?
Believe it or not, your estimated due date (EDD) is simply that – an estimate. The majority of women, between 95-96 percent, will give birth on a day other than their due date. On average, women give birth between 37 weeks and 42 weeks according to the standard method of calculation, called Naegele’s rule.
Naegele’s rule was first observed in 1744 by Herman Boerhaave, a professor in the Netherlands. He studied the records of 100 different pregnant women and observed that the average due date could be calculated by adding 7 days to the last menstrual period and then adding 9 months. The only issue that resulted was whether that was intended to be calculated from the first day or the last day of the menstrual cycle.
In 1812, the namesake of Naegele’s rule, German professor Carl Naegele, continued the observations that Boerhaave began, but he also failed to clarify when the count should begin – either when the last menstrual period began or when the last day of the last menstrual period ended. Because of this lack of definition, doctors would use the rule either way until an American publication in the 1900s made a more definitive stance in favor of counting from start and not the end of your last menstrual period. Unfortunately, there are inherently flawed concepts within this rule: a woman’s menstrual cycle is 28 days, ovulation takes place on day 14, and an embryo always implants at the same time in every woman.
Thankfully, due to advancements in ultrasound technology a more accurate method of calculating due dates began: early ultrasounds. According to studies, the way to get the most accurate due date is to have an early ultrasound between 11 and 14 weeks of gestation. This is beneficial to women – especially women who ovulate later – as this more accurate method can give a more realistic due date and reduce the risk of induction for being post term (when a women is over 42 weeks pregnant according to LMP). There remains to be some controversy over the safety of ultrasound use during pregnancy, so this might not be an option for everyone.
Not to fear, this is where other studies have proven extremely valuable. One such study observed that women birthing for the first time were likely to birth between 40 weeks and 5 days and 41 weeks and 2 days, while women who had already gave birth generally gave birth between 40 weeks and 3 days and 41 weeks. This shift can be especially impactful for women who have later than normal ovulation patterns – again to reduce the amount of induction for post term babies.
And still, another study conducted confirmed the median gestation to be 40 weeks and 5 days (285 days). These studies actually looked at the dates from the time of ovulation (confirmed through ovulation testing), to when the woman gave birth. This was a particularly fascinating discovery, since some women had actual gestational times as late as 45 weeks and 6 days from the LMP, but really they gave birth exactly 40 weeks and 4 days after ovulating! Not only that, but other factors affect gestation, even things like hormone reactions, making gestational times either longer or shorter, depending on the event.
So, what does all this poorly paraphrased information do for you? We are hopeful that it shows there are other factors behind your due date, and to 1) encourage you as you get closer to “late term” pregnancy – that this is normal and expected, 2) to share with you information that can help you better advocate for yourself, and, finally, and most importantly, 3) that you can honestly tell people you don’t know your due date and give them a “sometime around the end/beginning of X month.” And that’s just us being nice about it. You can be as spicy as you want *wink*.
For further research, definitely check out one of our favorite resources. You can deep dive into the studies yourself and check out the other wonderful information they provide.
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