Diagnostic Floow

Diagnostic Follow-Up In Prenatal Testing

If a screening test indicates a higher-than-average risk, your health care professional may want to perform a basic ultrasound, which can help determine the gestational age of the fetus and show if a woman is carrying twins. If either of these factors accounts for the abnormal triple screen test result, no further testing is needed. Otherwise, a more detailed ultrasound examination may be performed, which allows a look at the baby's brain and spinal cord, as well as other parts of the body. This ultrasound can often identify an open neural tube defect or other malformation associated with an abnormal screening test.

Your health care provider may suggest you consider either amniocentesis or chorionic villus sampling (CVS). Both are diagnostic tests to determine whether the fetus actually has the disorder in question. Amniocentesis is performed more frequently and should be the choice if you're at risk having a child with neural tube defects.

If you have amniocentesis, a doctor will use a needle to withdraw a sample of amniotic fluid (the fluid surrounding the fetus) for analysis. The procedure is typically performed during the second trimester, at 15 to 18 weeks.

Amniocentesis or CVS are also offered for high-risk pregnancies, which could be indicated by any of the following:

· you will be 35 or older at delivery

· your family has a known genetic disorder

· you have had a previous child with a birth defect

· you and your partner are carriers of the same recessive disorder

Both CVS and amniocentesis can cause cramping, and a small number of women have miscarriages following the procedures (the risk is higher with CVS, but still only about one percent).

CVS is an alternative to amniocentesis. The procedure is done at 10 to 12 weeks and involves analyzing a sample of placental tissue. A thin tube inserted through the vagina and cervix is used to suction out the tissue sample. However, unlike amniocentesis, CVS cannot be used to test for neural tube defects, such as spina bifida and anencephaly. Therefore, it's usually recommended that a woman undergoing CVS also have the prenatal blood test called the maternal serum alpha fetoprotein (MSAFP) screening test, at about 16 to 18 weeks of pregnancy. This test identifies most (but not all) pregnancies at risk for neural tube defects.

If you have a choice of amniocentesis or CVS, discuss the risks and benefits of each and the possible results with your doctor and, if possible, a genetic counselor.

CVS is more attractive to some women because it can be done much earlier, at 10 weeks to 12 weeks. Many women say that this is a time when pregnancy is still a private issue. But other women feel the slightly higher risk of CVS is unacceptable, and they wait for the amniocentesis.

Amniocentesis and CVS are used to diagnose many, but not all, genetic disorders prenatally. If a genetic disorder for which a test is available has surfaced on either side of the family, or if you and your partner are carriers for the same testable recessive disorder, your health care professional will probably suggest testing, to determine if the fetus has inherited the disease. If two parents are carriers for the same recessive disorder, each child has a 25 percent chance of being born with the disease.

Of course, the ideal time for many tests is before you get pregnant. If you are planning a pregnancy, preconception genetic counseling sessions can help determine what, if any, conditions may be in your genetic background that future children could inherit. Preconception screening offers an opportunity to make decisions without the pressure of an advancing pregnancy. You can also start to take folic acid (4 mg/day) before conception to reduce your risk of having a baby with a neural tube defect.

Handling The Results

No test guarantees a healthy baby. While amniocentesis and CVS are extremely accurate, they look only for evidence of specific genetic or chromosomal disorders and cannot take into account other factors, including sporadic genetic mutations (without a hereditary component), which may affect a child's development.

When an untreatable genetic disorder is diagnosed prenatally, parents have the option of continuing or terminating the pregnancy. A genetic counselor can help you learn more about the disorder and weigh your options in a neutral setting. The decision is a tough one, and parents may weigh such factors as whether the disorder can be treated, the family's ability to manage the disorder or disability, the extent to which an affected child will be disabled or in pain, and how long the child is likely to live.

Prenatal testing can be valuable if you opt to continue a pregnancy knowing the child will be born with a particular disease. The diagnosis often can help you, your family and your health care team better manage the pregnancy, the delivery and any treatment the newborn will need.

In rare cases, a disorder diagnosed prenatally can be treated before birth. For example, congenital adrenal hyperplasia, which causes genital abnormalities in girls, can be treated with hormones given to the mother. Some centers are also experimenting with in utero surgery to correct spina bifida. But such opportunities to correct birth defects are still very much the exception.