What Are The Prenatal Tests ?

Prenatal tests are used to determine whether the fetus has a congenital defect are becoming increasingly common, particularly for women older than 35 years. Here are some of the prenatal tests now in use:

Genetic amniocentesis: is performed early in the second trimester of pregnancy, usually between the 16th and 18th weeks of gestation. It is recommended for women 35 or older, for those who have a family history of congenital abnormalities such as Down syndrome or spina bifida, and for women who have previously given birth to an abnormal child.

During amniocentesis, your abdomen is anesthetized with a local anesthetic and the physician, guided by ultrasonography, inserts a long, thin needle through it into your uterus. This punctures the amniotic sac in which the fetus floats. A small amount of amniotic fluid is drawn into the needle, which is then removed and sent to the laboratory for tests.

This fluid contains valuable genetic information about your growing baby. After a few weeks in a culture medium, the cells can be studied for chromosome distribution. The presence of Down syndrome, indicated by the presence of an extra chromosome, can be successfully diagnosed with amniocentesis. In addition, other defects such as neurological problems, kidney diseases, and metabolic disorders may be diagnosed.

Your baby’s sex also can be determined, which can be helpful if there is a risk of a sex linked disorder. Amniocentesis also can be used later in pregnancy to detect Rh disease or to ascertain whether the lungs are developed enough to withstand an early delivery. Amniocentesis is usually safe for mother and child, but there is a slightly increased risk of miscarriage after the procedure.

Chorionic villus sampling: is similar to amniocentesis in that it is an invasive technique that can detect many genetic abnormalities, including Down syndrome. A big advantage is that it can be done between 6 and 8 weeks of gestation. Thus, if a defect is found, you have the option of terminating the pregnancy in its early stages. During chorionic sampling, a small piece of the placenta supporting the embryo is removed. Using ultrasonography, the physician inserts a catheter or small tube into your cervix. A suction procedure is used to suck the tissue into the catheter. You may feel slight discomfort but little more than is normally felt during a pelvic examination.

The removed tissue contains the same chromosomes as your developing baby. After an analysis of the membrane, your physician can determine whether your child has Down syndrome or other chromosome diseases. The test also can determine sex. Chorionic villus sampling is not as effective as amniocentesis for detecting some disorders, however. Sometimes amniocentesis is used after chorionic villus sampling, especially when it becomes important to rule out one of these defects. Like amniocentesis, chorionic sampling slightly increases the risk of miscarriage. Moreover, the risk of intrauterine infection is greater than that associated with amniocentesis.

The maternal alpha fetoprotein: analysis is a noninvasive blood test that is useful for detecting defects in the nervous system, sometimes Down syndrome, and some other chromosome abnormalities. Because the test carries no risk to mother or baby, many physicians recommend that all their pregnant patients undergo the test for alpha fetoprotein at 16 weeks gestation. Alpha fetoprotein is a protein made by every fetus. Normally, a small amount crosses the placenta and can be detected in the mother’s blood.

When it is found at higher than normal amounts, however, the fetus may have a neurological defect such as spina bifida (open spine) because large amounts of alpha-fetoprotein are allowed to leak into the amniotic fluid. These defects are rare, occurring in an estimated 1 to 2 babies out of 1,000. Your physician will draw blood from your arm for this test. To enhance the accuracy of the test, it should be done around your fourth month of pregnancy. If the test results are negative, the chances are that your baby does not have a neurological defect, although the test is not an absolute guarantee.

Approximately 50 women out of 1,000 have positive results. If this should happen to you, do not assume the worst. This finding may mean the fetus is simply older than was thought, that you are carrying twins, or that something else increased the alpha fetoprotein level in your blood. A second blood test is then warranted. When two blood tests are positive, an ultrasound examination usually is done. If this test fails to find a defect, amniocentesis is the next step. Of 50 women with positive test results, only 1 or 2 will be carrying a child with a defect of this kind.

Percutaneous umbilical cord sampling: tests fetal blood from the umbilical cord for the prenatal diagnosis of some genetic disorders. It is done most easily in the third trimester. This technique also has been used to determine oxygen and carbon dioxide levels in the blood, which may help diagnose growth retardation.

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Yasser Elnahas

MD, PHD, Professor Of CardioVascular Surgery
Dr. Yasser Elnahas, Is an associate Professor of Cardiovascular Surgery. Dr. Elnahas was trained as a fellow At Texas Heart Institute And Mayo Clinic Foundation.Dr. Elnahas is dedicated to educating the general public about different disease conditions and simplifying the medical knowledge in an easy to understand terminology.

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