Womb With A View
Ultrasound exams are routine during pregnancy, but how much do you actually know about them? Here’s how they work, and what they can and can’t tell you.
Among the most memorable pregnancy moments are those when you see your developing baby on the ultrasound monitor. That flickering heartbeat! Those tiny fingers and toes! But ultrasound exams (aka sonograms) can also be mysterious and nerve-wracking. “Some women come in thinking nothing could be wrong, and others come in worrying about everything,” says Joanne Stone, M.D., a professor of obstetrics, gynecology and reproductive Science at Mount Sinai School of Medicine in New York. Fortunately, she adds, most women have totally normal ultrasounds, and even when there is an unusual finding, it often just suggests a slightly increased risk of a rare condition. And in the minority of cases when something serious is suspected or detected, ultrasounds can provide essential information before a baby is born. To help demystify the process, here are answers to the most common questions about ultrasound exams.
What exactly is an ultrasound? High-frequency sound waves are sent into your uterus, where they bump into the fetus’s tissues and echo back; a computer analyzes the echoes and transforms them into black-and-white images on a monitor. During a typical exam, a gel is spread on your abdomen to help conduct the sound waves, and the doctor or, more often, an ultrasound technician glides a transducer around your belly while watching and capturing the images. She’ll likely give you a talking tour of your baby’s body and capture several hard-copy photos for you and the doctor.
When is the first ultrasound, and what does it look for? This is typically performed around week six or seven (usually at your first prenatal visit) to locate the heartbeat and estimate the age of the fetus. “Most women are more excited than nervous during this exam,” Stone says. It’s also done to rule out multiples or an ectopic pregnancy, one that develops outside of the uterus. Sometimes a vaginal ultrasound is performed at this early stage. For this, you lie back as if you’re having a pelvic exam and a narrow wandlike device is inserted; the procedure is painless.
When and why are other ultrasounds usually performed? Typically, an ultrasound is done between weeks 11 and 13 as part of the first-trimester screening for chromosomal abnormalities, including Down syndrome. (The other part of the screening consists of a blood test.) The nuchal translucency, or NT, test uses ultrasound to measure the clear (translucent) space in the tissue at theback of the fetus’s neck. Babies with chromosomal abnormalities tend to have more of this tissue than others. However, Stone says, “This is just an indication of risk, not a diagnosis of Down syndrome.” For a definitive diagnosis, a follow-up procedure such as amniocentesis is required. This ultrasound also looks at the size and position of the placenta and the level of amniotic fluid.
Another commonly performed ultrasound is the anatomical survey, done at weeks 18 to 20. This is a head- to-foot scan that looks for any abnormalities or growth problems. “Women tend to be nervous during this scan because they don’t know what may be found,” Stone says. “But when I do find something, I’m usually able to reassure a patient,” she adds. For example, if a baby’s thighbone is shorter than normal, it most often is simply a reflection of having short parents rather than a sign of any abnormality.
An ultrasound is often done late in pregnancy to confirm that the baby is head down and to estimate his size.