What it is
The ultrasound exam may be the prenatal test you've heard the most about. Your health care provider can use ultrasound imaging to get a picture of your unborn baby and determine how your pregnancy is progressing. Usually, you're able to watch the screen and see images of your baby while the test is being done. Ultrasound can also be used to diagnose some types of birth defects, such as a spinal abnormality (neural tube defect) or, in some cases, a heart defect.
Using ultrasound, this first-trimester test measures the size of a specific region under the skin behind your baby's neck. An increase in the size may be an indication of Down syndrome, a birth defect of the heart (congenital heart disease) or other abnormalities. Health care providers aren't exactly sure why this is. They suspect that it may be because of a buildup of lymph fluid due to underdevelopment of the lymphatic ducts. A rigorous approach to standardizing this measurement is needed for the test to be as accurate as was reported in early studies.
A third option in prenatal testing is called pre-implantation genetic diagnosis. If you choose to have your eggs fertilized outside of your body — a procedure called in vitro fertilization (IVF) — doctors can perform genetic tests on the embryo before implanting it in your uterus. The procedure is complex. First, fertilization of the harvested eggs occurs in the laboratory, creating several embryos. A single cell is removed from each embryo, and genetic analysis is done. Only the embryos without evidence of genetic disease are selected for implantation.
To use this test, the couple must be at an increased risk of having a child with a testable condition. Both parents must be tested for genetic problems so the health care provider knows what problems to look for (see "Genetic carrier screening" on page 269). The test is designed to look for genetic defects that are more likely to occur, such as genetic mutations or balanced chromosome rearrangements. It may become useful in finding conditions such as sickle cell disease or cystic fibrosis.
The ultrasound exam works by directing sound waves of very high pitch at the tissues in your abdominal area. These sound waves, which you can't hear, bounce off the curves and variations in your body, including the baby in your uterus. The sound waves are visually translated into a pattern of light and dark areas. The waves create an image of the baby on a monitor. This is similar to shouting into the Grand Canyon and being able to hear the echoes come back to you as they bounce off the peaks and valleys of the canyon.
Several different types of ultrasound examinations are available:
• Standard ultrasound. This ultrasound creates two-dimensional (2-D) images that can give your health care provider information about your pregnancy. It may show the gestational age of your baby, how he or she is developing, and the relationship between your body and the baby. It usually lasts about 20 minutes.
• Advanced ultrasound. This is also called a targeted ultrasound. It's often used to explore a suspected abnormality found during a standard ultrasound or a triple test. The exam is more thorough and may use more sophisticated equipment. It's also longer, taking from 30 minutes to several hours. Your health care provider may recommend an advanced ultrasound if you're considered to have a high-risk pregnancy. Advanced ultrasound is a noninvasive approach that may add further information where the only alternative is genetic amniocentesis. This type of ultra sound can be used to view your baby's head and spine in detail, and it's 95 percent effective at diagnosing neural tube defects.
• Transvaginal ultrasound. In early pregnancy, your uterus and fallopian tubes are closer to your vagina than to your abdominal surface. If you have an ultrasound during your first trimester, your health care provider may opt for a transvaginal one. It'll provide a clearer picture of your baby and the structures around it. A transvaginal ultrasound uses a slender, wand-like device that's placed inside your vagina. It sends out sound waves and gathers the reflected information.
• Three-dimensional (3-D) ultrasound. This newer type of ultrasound offers 3-D images with details similar to those of a photograph. It's used in selected medical centers to enhance the understanding of images from an advanced ultrasound.
• Doppler ultrasound. Doppler imaging measures minute changes in the frequency of the ultrasound waves as they bounce off moving objects, such as blood cells. It can measure the speed and direction at which blood circulates. With it, health care providers can determine how much resistance there is to the flow of blood through various tissues.
If you have high blood pressure, a Doppler ultrasound may help determine whether blood flow is being limited for the baby or the placenta. This may help inform your health care provider about the effect of high blood pressure or other stresses on the baby.
• Fetal echocardiography. This type of exam uses ultrasound waves to provide a more detailed picture of your baby's heart. It focuses on the heart's anatomy and function. It may be used to confirm or rule out a congenital heart defect.
When it's administered
An ultrasound exam can be done at any time during your pregnancy. Most health care providers obtain an ultrasound between the 18th and 20th weeks. By this time, the fetus is big enough to be evaluated yet not so big that gestational age assessment is inaccurate. And your baby has developed enough so that structural problems can be detected and all four chambers of the heart can be seen.
In some situations, such as a high-risk pregnancy, ultrasounds may be repeated throughout the pregnancy. They're used to monitor the health of both the mother and the baby and to track the baby's growth.
How it's done
You'll probably be asked not to empty your bladder before the exam, especially if the ultrasound is done early in your pregnancy. A full bladder eliminates pockets of air between the uterus and the bladder, which can distort the sound waves and produce an unclear image. If you're having a transvaginal ultrasound or if it's late in your pregnancy, a full bladder isn't necessary.
A gel is applied to your abdominal area. (You may want to wear a two-piece outfit to make it easier to expose your abdomen.) The gel acts as a conductor for sound waves and helps to eliminate air bubbles between the transducer and your skin. The transducer is a small plastic device that both sends out the sound waves and records them as they bounce back.
During the exam, the ultrasonographer moves the transducer back and forth over your abdomen, directing sound waves into the uterus. The sound waves reflect off bones and tissue. As the transducer captures the reflected sound waves, they're digitally converted into black, white and gray images on a screen. The images may be somewhat hard for an untrained observer to decipher, so don't worry if you can't see your baby. Ask your health care provider or technologist to help explain what's on the screen.
Depending on your baby's position, you may be able to make out a face, tiny hands and fingers, or arms and legs. Throughout the exam, your health care provider will stop to measure the baby's head, abdomen and thigh bone, among other structures, to record growth. He or she may also take some pictures to document important structures. You'll probably be given copies of some of these scans. Some clinics may also offer you a videotape of the ultrasound.
Based on the images produced by the ultrasound exam, your health care provider can determine a number of things about your pregnancy and your baby, including:
• The fact that you are indeed pregnant.
• How many weeks it has been since conception (baby's gestational age).
• How many babies you're carrying.
• Your baby's growth rate and development.
• Your baby's movement, breathing and heart rate.
• The sex of your baby. Being able to determine what sex your baby is depends on the baby's position in the uterus and the position of the umbilical cord. Decide in advance if you want to know this information.
• Structural variations or abnormalities in your baby, such as spina bifida.
• The location and development of the placenta. Sometimes a pregnancy develops outside the uterus (ectopic pregnancy), usually inside a fallopian tube.
• Whether you've had a miscarriage.
• Assessment of the cervix and the tendency toward preterm delivery.
• Measures of fetal well-being such as urine output, muscle tone and activity.
Ultrasound examination doesn't involve radiation. Forty years of experience suggests that it's a safe exam for both you and your baby.
Ultrasound is used so often with pregnancy that you may assume it's a routine part of prenatal care. But researchers have found that for most healthy women with normal pregnancies, a routine ultrasound doesn't seem to make a difference in the outcome of the pregnancy. It may not be cost-effective if there aren't any questions of normal fetal development.
If concerns do develop, those concerns are often best addressed by an ultrasound. If you're not sure when you became pregnant, an ultrasound can determine the baby's gestational age. If blood tests indicate an abnormality, an ultrasound may be able to identify it. If there's any bleeding or a concern about the baby's growth rate, an ultrasound is the best initial test. In addition, ultrasound imaging can be used to guide your health care provider while performing other prenatal tests, such as amniocentesis or chorionic villus sampling.
Many women and their partner look forward to an ultrasound because it gives them a first glimpse of their baby. Some parents value an ultrasound for finding out the sex of their baby. Although this is often possible, it's not recommended as the sole purpose of an ultrasound. Talk to your health care provider about your need for an ultrasound.
If results of the ultrasound are normal, no additional ultrasounds may be required. If your health care provider suspects abnormalities, he or she may recommend further testing. An advanced ultrasound, amniocentesis or other studies may be used to confirm or rule out a diagnosis. In many higher-risk pregnancies, ultrasounds are used frequently to monitor the pregnancy.
Although ultrasound is a very useful imaging tool, it can't detect all fetal abnormalities. If an ultrasound can't offer an explanation for a perceived problem, your health care provider may recommend other diagnostic testing, such as amniocentesis or chorionic villus sampling.
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