Prenatal Testing Guide

Prenatal Testing Guide

All About Genetic Testing
By Adrienne Lieberman

Which genetic tests are available and who should have them?

Early in your pregnancy you'll need to consider whether to undergo diagnostic testing for the presence of certain genetic abnormalities. Your risk for such problems depends in part on your family history, your own history of previous miscarriages or stillbirths, and your age.

Genetic Risks

The risk for bearing a child with a genetic defect rises with age. At age 35, a woman's risk of having a baby with a chromosomal abnormality is one in 196.

Because that number approximately equals the one-in-200 risk of miscarrying as a result of amniocentesis, insurance companies typically cover genetic testing for maternal age-related problems only for women who are 35 and older.

The tests used for genetic screening carry some risks. You should know exactly what these tests can and can't do. For example, genetic tests can't rule out birth defects from environmental agents (fetal alcohol syndrome, for example) or from unknown causes.

Before agreeing to genetic screening, you should get detailed counseling from your doctor or from a counselor at the center where you'll take the test.

Before Testing

Typically you'll give the counselor the family history for both yourself and the father. Certain ethnic groups are associated with an increased risk for hereditary disorders, as in the cases of cystic fibrosis in Caucasians of northern European descent, Tay-Sachs disease in Eastern European Jews, and sickle cell disease in African-Americans.

Then your counselor will probably go over what chromosomes are, what's normal, and what's not. You should learn how the analysis will be handled, the complications and potential risks, and any restrictions on activity after the tests.

Unfortunately, for the rare woman who gets bad news after genetic testing, treatments exist for only a few of the serious conditions that can be identified. While a handful of women use genetic testing to prepare themselves to care for a disabled child or so that special precautions can be taken after the birth, most women consider genetic testing only if they would consider terminating a pregnancy.

About the Tests

Either of the following two tests--chorionic villus sampling or amniocentesis--can take a genetic picture of your baby. If you have one of these tests, you'll most likely get the happy news that your baby's genes are normal.

What Is CVS?

In chorionic villus sampling (CVS) a sample of the chorionic villi--hairlike projections that grow outside the membranes surrounding the fetus--are removed for genetic analysis. The procedure usually takes less than half an hour.

An Rh-negative mother should get a shot of RhoGam, a substance that will reduce her likelihood of developing antibodies that attack the baby's blood.

After using ultrasound to position the sampling device correctly, the doctor passes a needle through the abdomen (transabdominal CVS) or inserts a slender catheter through the vagina and cervix in order to withdraw the chorionic villi from the uterus. Results are available in a few days.

What Are the Risks of CVS?

Some studies have linked CVS to an increased rate of limb deformities, particularly if the test is done before the 10th week of gestation. (Though CVS can be performed as early as eight weeks, it's typically done between 10 and 12 weeks.)

CVS carries approximately a 1.5 percent risk of miscarriage--compared with .5 percent for amniocentesis--because of inadvertent rupture of the amniotic sac. The risk of miscarriage is related to the skill of the operator.

Because CVS is performed early in pregnancy, when miscarriage is more likely anyway, a miscarriage that follows CVS can't automatically be attributed to the procedure.

The small number of parents who learn via CVS that their fetus is genetically abnormal have the option of an earlier, safer abortion than would be available after amniocentesis, which is performed later in pregnancy. On the other hand, unlike amniocentesis, CVS cannot diagnose neural tube defects.

Because CVS is available in only a few medical centers, it may not be covered by your insurance, so be sure to ask.

What Is Amniocentesis?

During amniocentesis, ultrasound guides the doctor as she passes a needle through your abdomen and into the amniotic sac. Approximately a teaspoonful of amniotic fluid is withdrawn, then analyzed for the presence of alpha-fetoprotein (to detect neural tube defects) and cultured to detect genetic abnormalities. Results are available in one to three weeks.

Amniocentesis for genetic testing is typically performed between 13 and 18 weeks, though a few centers are experimenting with doing it in the 11th or 12th week. Rh-negative women should get a shot of RhoGam.

In addition to genetic testing in early or mid pregnancy, amniocentesis may be performed later. If labor starts prior to 37 weeks, for example, late amniocentesis can be used to detect lung immaturity, which commonly poses a threat to premature babies.

All About Ultrasounds
By Terri Isidro-Cloudas

Your first look at your developing baby

If you're pregnant, chances are you'll have an ultrasound at some point. Read the answers to these commonly asked questions to learn what ultrasounds are, why they're necessary, and what they feel like.

What's an ultrasound?

Ultrasounds help doctors see internal fetal and maternal structures. The ultrasound probe, or transducer, goes either on the mother's abdomen or, less frequently, inside her vagina. The transducer transmits high-frequency sound waves that echo back and are transformed into a picture on a video screen. This picture shows the fetus inside the womb. Often, parents will be given a printout of the ultrasound to keep.

At six weeks' gestation, it's possible to see the baby's heartbeat. For many expectant parents, it's an added bonus that an ultrasound given after 20 weeks can sometimes identify the sex of the baby. However, in some cases it's not possible to see the baby's genitalia and parents are kept guessing.

Why an ultrasound?

Ultrasounds have many applications in pregnancy, and up to 60 percent of expectant mothers receive at least one scan. According to the American College of Obstetrics and Gynecology, ultrasounds are usually prescribed for the following reasons:

To establish the baby's age. Ultrasound helps accurately date a pregnancy, preventing unnecessary interventions. About 30 percent of pregnancies have dating errors. For dating purposes, an ultrasound scan should be performed prior to 20 weeks; it's typically scheduled between 12 and 16 weeks.
To see the baby's heartbeat. In late pregnancy, Doppler ultrasound enables a technician to measure circulation in the placenta and umbilical cord. This technique, called velocimetry, evaluates fetal well-being in pregnancies that are at risk because of growth retardation in the baby or high blood pressure in the mother.
To identify fetal abnormalities. Because it allows the physician to see the baby's developing body in great detail, an ultrasound is key in the detection of fetal abnormalities.
To determine the position and size of the baby. With an ultrasound, a doctor can see a baby in breech position and determine if the baby is too big to fit through the mother's pelvic cavity.
To detect multiple pregnancies. Scans help confirm multiple pregnancies and assess the babies' growth and health.
To perform the genetic test chorionic villous sampling, or to obtain a blood sample from the baby. An ultrasound helps safeguard certain genetic-testing procedures by allowing the doctor to ascertain the safest place to insert a catheter or needle.
To assess pregnancy complications such as bleeding or pain. Ultrasound helps the physician see the source of the bleeding and identify possible complications such as placenta previa.

Is there any preparation for an ultrasound?

After you make your appointment, your doctor will give you an instruction sheet telling you to what you need to do. Generally, women who are less than 14 weeks pregnant will be asked to fill their bladder to capacity. Sound waves travel better through liquid, so a full bladder improves the quality of an ultrasound during early pregnancy. As a woman's pregnancy progresses, a full bladder is not as essential because the uterus and fetus are so large. But even then, some physicians ask their patients to come with a full bladder because transmission of the sound waves is better with fluid in the bladder.

What does an ultrasound feel like?

The process is not painful. When you have an abdominal ultrasound, the doctor will smear a clear gel on the skin of your abdomen to help the device pass smoothly over the skin. Some doctors warm the gel. Otherwise you might get a chilly sensation from the cold gel on your skin. The doctor usually applies some pressure as she moves the transducer device along your abdomen. The transducer transmits sound waves that create a picture of the baby inside. If you're ticklish, you might find yourself challenged during this procedure. Take a deep breath and try to relax!

A vaginal ultrasound is done with a wand-shaped probe covered with a latex sheath (like a condom). The doctor will apply some lubricant and gently insert the ultrasound into your vagina. She will move the device to form the picture she needs on the ultrasound screen. The procedure doesn't hurt, but you might find it uncomfortable in the same way you might find a pelvic exam uncomfortable. Again, relaxation and focus are key. And don't be afraid to tell your doctor if you need her to stop and give you a minute to breathe.

One of the best parts about having an ultrasound is actually seeing your baby while he's still inside you. For many women, this is a turning point during which they feel even more bonded to the life they're nurturing.