The Pregnancy Diaries: Measuring Big or Small For Gestational Age

By Ana-Marija Dolaskie — Sep 21, 2017
At her last appointment, ACSH's Ana Dolaskie was told she is measuring small for gestational age? What does this mean? Since it could mean many different things, let's clarify. 

At my last two OB appointments (34 and 36 weeks along) I was told that I am measuring 1- to 2cm smaller for gestational age. 

"You're petite, so it's not surprising," reassured my doctor. "But let's do an ultrasound next week to look at baby's growth."

Next week? That feels like an eternity! As an expectant mother, waiting a week to confirm if everything is OK can certainly be anxiety-filled. So here's a bit of clarity on what it all means.

Gestational age — the age of the baby — is typically calculated from the first day of the mother's last menstrual period. Since the exact date of conception isn't always known, the most accurate gestational age and due date is calculated between the 8th and 18th weeks of pregnancy, using an ultrasound. 

Once the due date is determined, a doctor will measure the belly of a pregnant woman at each appointment, starting in the second trimester. This is done by running a tape measure from the upper part of the pubic bone, to the top of the uterus, or womb, to measure the fundal height. The number of centimeters the belly bump measures should be the same as the number of weeks mom-to-be is pregnant, give or take 2cm. 

Of course, there are perfectly normal reasons why the belly doesn't always measure exact, like carrying twins, being overweight/underweight, wrong due date, or simply having a small stature. 

A more serious reason to measure large for gestational age is polyhydramnios — or excess amniotic fluid. If you're not already carrying multiples, in this case, your OB will want to rule out:

  • maternal diabetes: gestational diabetes that isn't being managed properly can be the cause of excess amniotic fluid. 
  • genetic abnormalities: babies with very high fluid levels are more likely to have a genetic abnormality, like Down Syndrome, or other birth defects.
  • fetal anemia: caused by an Rh blood incompatibility or an infection such as fifth disease, fetal anemia can be treated with a blood transfusion in utero. The condition is rare, thanks to the RhoGAM shot — a triumph in medicine over Rh (rhesus) disease. We've written about it here. 

If you are measuring small for gestational age, the OB will likely perform an ultrasound to confirm the due date, and to rule out intrauterine growth restriction. There are several reasons for this condition:

  • abnormalities in the placenta: the placenta delivers oxygen and nutrients to your baby. If it's too small, detached, and not working properly, it may not be delivering the proper nutrients to your baby.
  • chromosomal abnormalities: Down syndrome, birth defects, or kidney problems could attribute to this.
  • smoking, drinking, or drug abuse: moms-to-be who engage in dangerous behaviors like these could experience problems with fetal growth.

This article isn't meant to scare moms-to-be, myself included, but it's important to know and understand the various reasons your OB may want to take a closer look. Of course — more often than not — measuring slightly larger or slightly smaller for gestational age means your baby has dropped and you have a perfectly healthy chubby or perfectly healthy petite baby!







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