Why Do Doctors Change Due Dates: 4 Facts You Must Know

You’ve saved your due date on every single calendar at home. You’ve shared it with your twitter followers. But now, you have your doc telling you the date’s no good. So what’s going on?

Math for pregnancy isn’t always accurate because, at the end of the day, unless you went down the IVF road, your doctor can’t really identify when exactly you have conceived.

Due to this very fact, your pregnancy’s due date is calculated using day one of your last period. From that very date, your doctor or midwife will count 40 weeks of pregnancy, then officially mark the estimated due date on a calendar. However, sometimes this date is altered. Here are the common reasons:

In This Article

1. Had your first ultrasound in your second trimester?

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Did you happen to get one during the second trimester? This could be a probable reason as to why your due date seems to be uncertain. According to the American College of Obstetricians and Gynecologists, “Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks gestation) is the most accurate method to establish or confirm gestational age” [1].

Those ultrasounds that happened to be performed in the first-trimester fall between a 3-5 days accuracy. If you were wondering when is the most accurate time during the pregnancy to get a dating ultrasound, then it’s anytime between the first 8 to 11 weeks. This is when your fetus will be growing rapidly hence there will be massive differences in the development with each week.

2. Suffer from irregular menstrual periods?

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It can be fairly difficult to calculate a due date for soon-to-be mommies using the “date of the last menstrual period” method if they happen to be having irregular cycles. This is the very reason why an ultrasound exam will be prescribed in order to calculate your precise gestational age.

It is important to note that several pregnancies can be confirmed not using an ultrasound but a Doppler, heartbeat monitor. If this was how your due date calculation played out, then it could explain why your date’s not all that accurate.

3. Abnormal AFP levels alert!

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Between your 14th and 22nd week of pregnancy, your doctor will most likely advise you to get a blood test done in order to measure your alpha-fetoprotein, also known as ‘AFP,’ levels. AFP is produced by the liver of your unborn child. Generally, this is part of your triple screen: AFP, a type of estrogen (unconjugated estriol) and human chorionic gonadotropin (hCG).

AFP levels can help your doctor determine whether your baby is suffering from any birth defects such as anencephaly or spina bifida. It can also screen for Edwards syndrome (trisomy 18) or Down’s syndrome [2]. If you are shown to have high AFP levels, it could possibly indicate birth or genetic abnormalities or maybe even a miscalculated due date since the levels happen to vary during the course of the pregnancy.

4. Have an above-average fundal height?

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With each visit to you doctor, he or she will proceed to measure your current fundal height, i.e. the distance measured from the top of your growing uterus to the top of the midline of your pubic bone. This measurement can roughly calculate the size of your uterus and hence correlate to the length of your pregnancy.

Your doctor could very well find that compared to the standard charts for growth your measurements don’t match up. If you happen to be showing more than a three-week variance, your doc will decide to change your due date in order to accommodate this change.

If a miscalculation of your due date is throwing you into panic mode, remember this: no matter when your baby is born, he or she will be absolutely beautiful. So enjoy the unpredictability- you’ll be facing it a lot more as a parent anyway!

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