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. Merely when it comes to determining a woman's due date, sometimes things can go a footling murky.

After all, you may have lost rails of your menstrual schedule or y'all might non exist certain which roll in the hay was the lucky ane. And unless you had in vitro fertilization (IVF), your dr. has no manner of knowing the exact time of conception either.

Because of this, pregnancy is calculated from the start mean solar day of your last menstrual period (or LMP). And from that day, xl-weeks of pregnancy is counted, and your doctor will bust out the red pen to marking your due date on the calendar —and you'll likely commit this day to retentiveness.

Sometimes, however, your due appointment may be revised. Here's more about why it happens so you tin 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, you due date is counted forrard with this timespan.

If you conceived using IVF, your due date will exist calculated using your date of transfer, taking into account whether a three- or five-day embryo was used.

Regardless of how you conceived, retrieve that your due date is ultimately just an estimate. Every pregnancy is unlike, with some babies jumping the gun and arriving early and others taking their own sweetness time and showing up belatedly.

Why might your due date change?

Your pregnancy will be filled with appointments, tests, measurements and more than, which means there's lots of fourth dimension for your practitioner to check on your progress and perchance revise your due date.

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Here are some of the more mutual reasons your baby'southward arrival 24-hour interval might have to change:

  • You have irregular periods. Does your monthly go MIA from time to time? It can be pretty difficult to make up one's mind a due date using the date-of-final-menstruation method for women with irregular cycles — which is why an ultrasound exam is frequently required to determine gestational age.
  • Your first appointment relied on Doppler. Many pregnancies are showtime confirmed by a Doppler heartbeat monitor, not an ultrasound. If this was you, information technology could be why your doctor's original timing was off. (An early on ultrasound, commonly done at about 6 to ix weeks, can give the nigh accurate due date estimation by measuring the size of the embryo or fetus.)
  • Your showtime ultrasound was in the 2nd trimester. Did you go your very get-go ultrasound in your second trimester? If and then, this is another reason why your due date has shifted. Ultrasounds performed during the get-go trimester (not the second) requite the virtually accurate due engagement predictions, according the American College of Obstetrics and Gynecology (ACOG).
  • Your fundal height is above boilerplate. At each date, you'll take a fundal measurement, which is the altitude from the top of your pubic bone to the tiptop of your uterus, and this number closely correlates to how far along yous are. It could exist that your uterus size doesn't sync up with the standard growth charts (i.e. you measure large). And if the measurements are off by iii weeks or more, in many cases your due engagement is moved to reflect this.
  • You accept aberrant AFP levels. Between weeks 14 and 22, your md will likely social club a blood examination to screen for AFP — short for Alpha fetoprotein, a protein that's usually produced by the liver and yolk sac of your baby-to-be; levels vary widely throughout pregnancy. (This is ofttimes role of the quad screen.) While high levels of AFP sometimes indicate a potential genetic aberration, the number one reason is merely your due date was miscalculated.

How to Calculate Your Due Date

How does a inverse due date bear on your prenatal care?

A change in your due date shouldn't impact your prenatal care very much. In fact, your routine should continue every bit usual, which ways you should keep every cheque-up and appointment on your schedule and relay any concerns or questions yous might have with your practitioner at each visit. And if your due date changes, endeavour not to let it throw you off balance.

Remember — any day your infant is born will be beautiful. There's really no style to know your baby's exact arrival date until he'due south here, even if yous schedule an induction (he could surprise you and go in that location get-go). Enjoy the chemical element of surprise and unpredictability — it'south great exercise for parenting!