Ultrasound: A Trimester-by-Trimester Guide

First Trimester

You missed your period. Your breasts are sore and you're exhausted. You take a home pregnancy test and a plus sign appears. Hurray! You're pregnant! Time to get "official" confirmation at the ob-gyn's office with a blood test and an ultrasound exam.

What to expect at your exam
Your first ultrasound, also known as a sonogram, will take place when you're around 6 to 8 weeks pregnant. When you schedule your appointment, be sure to ask whether you need a full bladder for the test. Sound waves travel better through liquid, so a full bladder can enhance the quality of your ultrasound. As your uterus and the fetus grow (and you have more amniotic fluid), a full bladder matters less.

At this stage, your baby is very small and your uterus and fallopian tubes are closer to your birth canal than to your abdomen, so your ob-gyn will conduct the test transvaginally to get a clearer picture. The test is painless. Your ob-gyn will place a thin, wand-like transducer probe, which transmits high-frequency sound waves through your uterus, in your vagina. The sound waves bounce off the fetus and send signals back to a machine that converts these reflections into a black and white image of your baby. It will be hard to see much in this first snapshot, but a clearer photo will come around 13 weeks, which is the ideal time to share your exciting news.

Why it's important
Your ob-gyn listens for your baby's heartbeat and estimates his age by measuring his length from head to bottom; the baby is tiny in the first trimester and is growing about a millimeter a day. From this test, your doctor will be able to determine a more accurate due date and track milestones during your pregnancy. Your ob-gyn will also rule out a tubal (ectopic) pregnancy, which is when the fetus grows in the fallopian tube instead of the uterus. (Don't worry: This occurs only 1 percent of the time.) And your doctor will also be able to tell if you're pregnant with multiples.

Other tests
All pregnant women are offered a nuchal translucency (NT) test, performed between 11 and 13 weeks, and this involves another ultrasound. The NT evaluates your risk of having a baby with Down syndrome, trisomy 18 (another chromosomal abnormality), or certain heart defects. In this two-part exam, a blood test measures levels of certain hormones and proteins in your body, and an ultrasound determines the thickness at the back of baby's neck (increased thickness indicates that he may be at risk for birth defects such as Down syndrome and trisomy 18).

At this stage, your ultrasound will likely be done on your abdomen. While you're reclining on an exam table, the doctor or ultrasound technician will slather gel on your belly, and then glide a plastic transducer over it. The test doesn't hurt, although the gel may feel cold and be messy. Wear two-piece clothing to your ultrasound, to allow for easy access to your tummy.

At some high-tech centers, doctors use 3-D ultrasound to provide pictures of the baby with photograph-quality details. High-tech centers may use this type of ultrasound to better evaluate a baby's growth and development, as well as detect facial abnormalities or neural tube defects.

Safety info
An ultrasound is considered safe for both you and your baby when it's used for medical purposes. A trained professional who can interpret the results with accuracy and who is a pro at detecting abnormalities should perform it. Your technician 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.

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