Due Date as a Reference Point
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Once you’ve confirmed your pregnancy, you might be curious to know your “due date” or when to expect your new baby’s arrival. (You will also likely be asked, “When are you due?!”, countless times over the next nine months.) It's easy to focus on this specific day. But did you know most babies are not born on their due date? In fact, the percentage of babies that are is in the single digits. So how is it calculated in the first place? And what’s the point of it exactly?
Calculating Your Due Date
A due date is calculated by counting forty weeks from the first day of your last menstrual period. Conception, at least in an average menstrual cycle, happens about two weeks after the start of a period. So a baby's gestational age is already about two weeks at the moment it is conceived! And most people won't know they are pregnant for at least a couple more weeks after that.
Your due date will be confirmed by your care provider, often at the first prenatal visit. They will ask you if you know the date of the first day of your last menstrual period (LMP). If you do, this will be used to establish a due date. An ultrasound done during early pregnancy (between 11 and 14 weeks for greatest accuracy) can also be used to establish or confirm this if you aren’t sure of the date of your LMP. Also, if there is a discrepancy between LMP and ultrasound dating (>5-7 days), your due date will be adjusted to reflect what the ultrasound is showing.
As stated in this Committee Opinion from ACOG:
If pregnancy resulted from assisted reproductive technology (ART), the ART-derived gestational age should be used to assign the estimated due date (EDD). For instance, the EDD for a pregnancy that resulted from in vitro fertilization should be assigned using the age of the embryo and the date of transfer.
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Due Date as a Reference Point
It may be more helpful and realistic to think of a due date as a reference point for your pregnancy, rather than a guarantee of when your baby will arrive Some reasons why not to get too attached to a specific day:
Menstrual cycles vary
Ultrasound is accurate but still has a margin of error
Babies don’t usually arrive right on time
In calculating a due date, your care provider is trying to estimate when your baby was conceived. Based on that, there is now an educated guess as to how many weeks you are along in the pregnancy at any given time. Knowing this guides your prenatal care, as well as decision-making.
Labor that begins prior to 37 weeks of pregnancy is considered preterm. Premature babies are those that are born preterm, or prior to that 37 week mark, and can suffer from lifelong health problems. Preterm birth can even be life threatening for the baby, especially if born earlier on. At week 37, a pregnancy is considered “term”. And a pregnancy is considered term until reaching 42 weeks. It’s normal for labor to begin on its own at some point during this five-week window, so it’s really more of a “due month”.
Understanding this can help in managing expectations and frustrations at the end of pregnancy, especially if your due date comes and goes with no signs of your baby. This normal range can also help you in making decisions at the end of pregnancy regarding if and when to induce labor.
Stages of Pregnancy
Once you know your estimated due date, you’ll know how far long you are in your pregnancy. Those 40+ weeks of pregnancy are divided into three stages or trimesters:
First (Weeks 0-13)
Second (Weeks 14-28)
Third (Weeks 29-40+)
Final Thoughts
While your care provider will calculate your due date, it’s fun to figure this out on your own before that first appointment. Plus, it can help you remember the first day of your last menstrual period ahead of that first prenatal visit. Make sure to note it down, if you can remember it!
I have a Due Date Calculator and Due Month Calendar in my Free Resource Library. Click the button below to access them!