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  • Writer's pictureErin Stevens

When are you due?

People in the OB world love to make jokes about Labor Day, so I figured I'd better write about something related to labor today. Let's talk about due dates.

I hate the term "due date". Your pregnancy is not a college essay. You do not fail if you turn it in late. You do not get extra credit for being done early. A lot of obstetricians have encountered at least one patient who showed up at their office or hospital on their due date unscheduled and not in labor, expecting something to happen. At least on a weekly basis, I think of the time in residency that a patient came in in preterm labor at 28 weeks, and the expectant grandfather excitedly exclaimed, "The earlier the better!" (nope nope nope nope nope). Any time someone sends out or posts a pregnancy announcement that says something along the lines of, "The newest member of our family is coming (enter due date here)," I just kind of think to myself, "Yeah, we'll see."

Our initial method of calculating the due date uses the first date of the last menstrual period and counts 40 weeks forward. The last menstrual period is simply a marker of when ovulation (release of the oocyte/egg) likely occurred. For someone who has a textbook 28 day (4 week) menstrual cycle, ovulation generally falls right in the middle (about 2 weeks after the first day of bleeding). The due date, then, is a measure of about 38 weeks after ovulation. Many people do NOT have 28 day cycles. Those with longer or shorter cycles have later or earlier ovulation, generally ABOUT 2 weeks prior to when the NEXT period would have happened but NOT always exactly that (confused yet?). So if you go to the doctor for pregnancy care and your due date is changed based on an ultrasound, you likely ovulated at a different time than your period would have predicted because your periods simply don't fit textbook expectations because you're a unique human. (Unfortunately, ultrasound has a margin of error, which widens as pregnancy progresses, so ultrasound also can't give us a definitive measure of 38 weeks post-ovulation.)

Ovulation is an important time because after that is when the egg and sperm become friends - fertilization. Once they meet up, the next important step is implantation, when the fertilized egg attaches to the wall of the uterus. (It's estimated that about 50% of fertilized eggs never fully implant. A period occurs, and no one even knows what happened.) Depending on which study you're looking at, the timing of implantation can occur anywhere from 5-18 days after fertilization, with the average being about 8-10 days. By technical definitions, pregnancy (not necessarily LIFE, that's a whole different discussion and debate) begins after implantation. So if you think of your due date, 5-18 days after 2 weeks after 40 weeks before that is when pregnancy truly started. Due dates feel a little less special when broken down like this. Sorry. If you go to your doctor at "6 weeks" of pregnancy, you have technically been pregnant for likely less than half of that.

Only about 5% of pregnancies end on the due date, because, again, it is just an estimate based on average length of gestation. After 37 weeks, a pregnancy is considered to have made it to term. When I think of a baby coming "early", it's in the context of 37 weeks+ being the goal rather than the 40 week due date that most patients have in mind. When a patient states that their last baby was "2 weeks early", then, I need to sort out if they mean that they delivered prematurely at 35 weeks (2 weeks prior to term) or if actually it was a term pregnancy that delivered at 38 weeks (2 weeks before their due date). More commonly, the patient means the latter. This matters in the context of management and expectations for the next pregnancy. Likewise, a baby born after the due date isn't necessarily "late". After 42 weeks is what we consider "post-term".

Based on that, 37-42 weeks is generally an okay time to deliver. BUT HOLD ON. We don't encourage babies to come out at 37 weeks (ie plan an induction or C-section) unless there is a medical issue complicating the pregnancy. The pregnancy is considered to be at term, but optimally we want to give the baby a little more time if all is going well and the pregnant person is not in labor. A study published within the last couple of years suggested that 39 weeks is a great time for babies to be born, and some hospitals have started to allow induction of labor for any patient at this time if the patient desires. That's also when we generally schedule C-sections for those desiring or requiring that mode of delivery (without other medical factors to consider). Other hospital protocols only allow "non-medically-indicated" inductions (otherwise called "elective inductions", when there's not a medical concern behind the planned delivery) after 41 weeks. Some obstetric clinicians will only discuss induction of labor at or after 42 weeks. When there IS a medical issue at play, we have to weigh the risks and benefits of the baby being inside vs outside of the uterus as the pregnancy progresses and use the best available evidence to guide timing of delivery.

In addition to figuring out when we might need or want to deliver the baby, due dates give us a measure of how far along a pregnancy is at any given time, which helps us perform specific testing appropriately and track when a pregnancy should be hitting certain milestones. If complications develop in the pregnancy, including going into labor earlier than desired, it helps us know what kind of interventions to perform and when. If a pregnant person needs to be admitted to the hospital, we can plan WHICH hospital is best as possible NICU care by gestational age varies from hospital to hospital.

In the end, your due date (or 40 weeks post-textbook period or 38 weeks post-probable ovulation or 37ish weeks post-maybe implantation date) doesn't matter for the reason most people think it does but can matter for a bunch of other reasons.

Anyway, happy Labor Day! I hope you're not laboring too much.

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