Ultrasound Measurements: What's Normal?

You'll get some interesting stats about your baby during your ultrasound -- here's what all those measurements mean.
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At your prenatal ultrasound appointments, your sonographer is doing much more than counting fingers and toes -- she's taking lots of measurements of your baby to check on your child's growth, and make sure that everything's A-OK.

One of the most common measurements is crown-rump length, which is used in the first trimester, and measures your baby's size from head to tush. The reason it's important? "This measurement is extremely accurate in dating the pregnancy in the first trimester," says Michele Hakakha, M.D., an ob-gyn in Beverly Hills and author of Expecting 411. "It helps us determine if the size of the fetus matches up with the size it should be based on the patient's last menstrual period." If this number is considered off early on, that could indicate potential issues. "A smaller crown-rump length may signal an embryo that is not developing normally and may have a chromosomal problem, or it could mean that a woman's dates are off and she is not as far along as she thinks."

After the first trimester, your baby will also have measurements taken of the diameter and circumference of her head, the circumference of her abdomen, and the length of her femur bone.

Those measurements will continue to be made as the pregnancy progresses, to make sure that your baby is growing along a predictable curve. "The computer within the ultrasound machine is capable of taking all the size measurements of the head, abdomen and femur, to give an estimate of size based on weeks of pregnancy," says Dr. Hakakha. "For example, if a woman who is 28 weeks pregnant goes in for an ultrasound, most of her baby's measurements should be equal to 28 weeks, as determined by the machine."

As long as your baby's measurements are progressing along a normal growth curve, you have no reason to worry. "A cause for concern arises when measurements are either much larger or much smaller than anticipated," Dr. Hakakha says. "A smaller baby in the second and third trimesters, particularly a baby whose abdomen is measuring smaller, may not be getting everything he or she needs from its placenta. A larger baby, on the other hand, may signal the presence of diabetes or other conditions."

If your doctor thinks your measurements don't add up, she may request additional tests to determine if there's cause for concern -- for your health or your baby's. "Ultrasound is paramount in measuring the size and growth of babies because it helps practitioners become aware of other health conditions that may be hiding in a pregnant woman, allows us to initiate more testing, if necessary, and prepare for delivery," Dr. Hakakha says. These measurements will help ensure that you and your baby are healthy throughout the pregnancy and beyond.

You'll get your first glimpse of your baby at this test, but doctors will be ensuring that everything's growing just as it should.

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