When I was pregnant with my first baby, I assumed I would give birth after my due date. I had heard so many stories of moms having to wait (and wait and wait) for their first baby to arrive, and so I expected the same thing to happen to me.
It’s funny the things new mamas can convince ourselves of.
Suffice it to say, when my water broke one week and two days before my due date, I was S-H-O-C-K-E-D. I mean, it’s not that I didn’t know it could happen. But I wasn’t exactly prepared for it to happen when it did. And even though I knew that due dates could vary by a week or two, give or take, I was supposed to give birth after my due date, dammit.
You’d think I would have learned by my second baby, but I was still pretty surprised when I went into labor a week before my due date. I guess I never really learn.
Since then, I’ve thought a bunch about due dates and how the timing of it all works. I began to notice something weird, too. Many of my taller friends seemed to have pregnancies that really dragged out, whereas many of my shorter friends (myself included) seemed to have shorter pregnancies.
Lo and behold, it wasn’t just a theory! There’s actual research to back this up. A 2015 study published in the Journal of Obstetrics and Gynaecology found that there was a correlation between height and gestational length.
Specifically, they found that shorter moms (less than 165cm or 5ft, 4 inches) had shorter pregnancies than their taller counterparts, by about 0.6 or 0.7 weeks. These moms also tended to have more early term births.
Interestingly, as I dug deeper, I found that height isn’t the only factor that might influence gestational length.
Take these fascinating tidbits from a 2013 study conducted by the National Institutes for Health. First of all, the researchers remind us all that due dates vary a whole lot, and that’s totally normal. Only 4% of pregnancies last 40 weeks. And only 70% of mothers give birth within 10 days of their due date.
According to the researchers, there are probably quite a few factors that contribute to your due date. One such factor could even go as far back as your baby’s conception. It turns out that babies who embryos look longer to implant tended to be born later. And moms who had a later progesterone rise after implantation had shorter pregnancies.
Totally strange and super intriguing, right?
There’s more, too. The researchers found that older women tended to have longer pregnancies, and that each year of age added about one day to their gestational period. Women who were larger babies at birth themselves had longer pregnancies as well. For each 100mg of a mother’s birthweight, the researchers estimated an extra day of pregnancy.
How long your previous pregnancies are can also have an impact. In the study, women who previously had longer pregnancies tended to continue having long pregnancies (sorry, gals!), leading the researchers to conclude that women tended to have consistent pregnancy lengths overall.
Obviously, all of these things are just estimates and describe overall trends. So if your pregnancy experience was vastly different, or varied from baby to baby, that’s totally within the realm of possibility. There is just so much variation when it comes to due dates.
And the fact is, researchers still don’t really know what exactly triggers a baby to be born. There is a theory that labor is triggered by a protein excreted by a baby’s lungs when their lungs become mature. Another theory is that a “genetic switch” from the placenta sets forth the hormone flux that begins labor.
I think the bottom line is that no one knows for sure what determines gestational length or what exactly triggers labor, but there are definitely patterns out there that are worth noting. And the whole thing is absolutely fascinating and mega-cool.
Basically, women are freaking miraculous and amazing, and it seems like we’ve only scratched the surface in terms of the intricacies and badassery of the female human body. Rock on, mamas.