Ultrasound: A Trimester-by-Trimester Guide

Third Trimester

prenatal checkup

For many pregnant women, their last ultrasound takes place at their 20-week anatomy scan. If you've gone past your due date, your doctor may want to keep a close eye on your baby with fetal heart-rate monitoring and ultrasounds to assess the amniotic fluid levels. Other reasons for third-trimester ultrasounds include concerns about the health of the placenta and questions about whether your baby's growth is on track. Just think of those sessions as time spent getting to know your baby. Soon enough, you'll be looking right in his baby blues.

What to expect at your exam
Earlier in your pregnancy, your doctor may have asked you to hold off on peeing before your scan. Sound waves travel better through liquid, so a full bladder can enhance the quality of your ultrasound. Now that your uterus and the fetus are larger (and you have more amniotic fluid), a full bladder matters less. When you schedule your exam, ask your doctor about this issue.

Why it's important
Many moms-to-be don't need an ultrasound in the third trimester, but if you're over age 35 or your doctor wants to closely monitor your baby's growth, you may get one or more before baby is born. (Other reasons for third-trimester ultrasounds include low levels of amniotic fluid, bleeding, and pre-term contractions.) You'll also get a follow up scan if your cervix was covered by the placenta at your 20-week scan. In 95 percent of cases the placenta moves away from your cervix by your due date, but if yours is still covering the placenta (this is called placenta previa), your ob-gyn may recommend a cesarean section (C-section) delivery.

Other tests
If you suffer from gestational diabetes, your doctor may scan you with a Doppler ultrasound in the last weeks. A regular ultrasound uses sound waves to produce images; this one bounces high-frequency sound waves off circulating red blood cells to measure blood flow and blood pressure. The test will determine if Baby is getting enough blood.

Safety info
Ultrasounds are considered safe for both you and your baby when used for medical purposes. A trained professional who can interpret the results with accuracy and is a pro at detecting abnormalities should perform it. Your tech should be schooled in obstetrical ultrasound, preferably at a center accredited by the American Institute of Ultrasound in Medicine. Experts discourage getting 3-D and 4-D (moving picture) ultrasounds at fetal portrait studios in places like shopping malls, where untrained personnel may give out inaccurate information.

Copyright © 2011 Meredith Corporation.

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