One enormous belly over three trimesters for about nine months equals ten tiny fingers and toes in the end. From this perspective, you’d think pregnancy math should be pretty straightforward. But when it comes to determining a woman’s due date, sometimes things can get a little murky.
After all, you may have lost track of your menstrual schedule or you might not be sure which roll in the hay was the lucky one. And unless you had in vitro fertilization (IVF), your physician has no way of knowing the exact time of conception either.
Because of this, pregnancy is calculated from the first day of your last menstrual period (or LMP). And from that day, 40-weeks of pregnancy is counted, and your doctor will bust out the red pen to mark your due date on the calendar —and you’ll likely commit this day to memory.
Sometimes, however, your due date may be revised. Here’s more about why it happens so you can be prepared for a possible change in your delivery plans.
How are due dates usually calculated?
As mentioned, your due date is calculated from the first day of your LMP. And since most pregnancies last an average of 280 days, or 40 weeks, your due date is counted forward with this timespan. But because this practice assumes you have a regular menstrual cycle of 28 days with ovulation occurring on the 14th day, it is not an exact science — it doesn't account for an irregular cycle or variability in the timing of your ovulation, for example.
If you conceived using IVF, your due date will be calculated using your date of transfer, taking into account whether a three- or five-day embryo was used.
Regardless of how you conceived, remember that your due date is ultimately just an estimate. Every pregnancy is different, with some babies jumping the gun and arriving early and others taking their own sweet time and showing up late.
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Why might your due date change?
Your pregnancy will be filled with appointments, tests, measurements and more, which means there’s lots of time for your practitioner to check on your progress and possibly revise your due date.
Here are some of the more common reasons your baby’s arrival day might have to change:
- You have irregular periods. Does your monthly go MIA from time to time? It can be pretty difficult to determine a due date using the date-of-last-period method for women with irregular cycles — which is why an ultrasound exam is often required to determine gestational age.
- Your first date relied on Doppler. Many pregnancies are first confirmed by a Doppler heartbeat monitor, not an ultrasound. If this was you, it could be why your doctor’s original timing was off. (An early ultrasound, usually done at about 6 to 9 weeks, can give the most accurate due date estimation by measuring the size of the embryo or fetus.)
- Your first ultrasound was in the second trimester. Did you get your very first ultrasound in your second trimester? If so, this is another reason why your due date has shifted. Ultrasounds performed during the first trimester (not the second) give the most accurate due date predictions, according the American College of Obstetrics and Gynecology (ACOG).[1]
- Your fundal height is above average. At each appointment, you’ll have a fundal measurement, which is the distance from the top of your pubic bone to the top of your uterus, and this number closely correlates to how far along you are. It could be that your uterus size doesn’t sync up with the standard growth charts (i.e. you measure big). And if the measurements are off by three weeks or more, in many cases your due date is moved to reflect this.
- You have abnormal AFP levels. Between weeks 14 and 22, your physician will likely order a blood test to screen for AFP — short for Alpha fetoprotein, a protein that’s normally produced by the liver and yolk sac of your baby-to-be; levels vary widely throughout pregnancy. (This is often part of the quad screen.) While high levels of AFP sometimes indicate a potential genetic abnormality, the number one reason is simply your due date was miscalculated.
How does a changed due date impact your prenatal care?
A change in your due date shouldn’t impact your prenatal care very much. In fact, your routine should continue as usual, which means you should keep every check up and appointment on your schedule and relay any concerns or questions you might have with your practitioner at each visit. And if your due date changes, try not to let it throw you off balance.
Remember — whatever day your baby is born will be beautiful. There’s really no way to know your baby’s exact arrival date until he’s here, even if you schedule an induction (he could surprise you and get there first). Enjoy the element of surprise and unpredictability — it’s great practice for parenting!
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