Some genetic problems can be diagnosed before birth. You may want to be tested during pregnancy for some of these problems, especially if you or your partner have a genetic defect in your family, or if either of you have had another child with a serious inherited problem.

Some of the genetic disorders that can be diagnosed during pregnancy include cystic fibrosis, Duchenne muscular dystrophy, hemophilia A, thalassemia, sickle cell anemia,

polycystic kidney disease, and Tay-Sachs disease. It is also possible to test for spina bifida and Down syndrome before birth.

Doctors may want to do genetic tests if a woman has had two or more miscarriages. Multiple miscarriages sometimes indicate a genetic problem.

Women over thirty-four or so are also considered good candidates for genetic tests, because the incidence of some birth defects goes up with maternal age. Older fathers are likelier than younger men to have children with new dominant genetic mutations, that is, birth defects that haven't run in the family before.

Tests vary in their degree of invasiveness. Blood tests are very safe indeed, and can give potential parents and their doctor valuable information. Ultrasound is generally considered safe, and can be very useful.

Some other tests are more invasive. Although they are as safe as modern medicine can make them, they are not tests that a doctor would recommend or a pregnant woman would want out of mere curiosity. Such tests include amniocentesis and chorionic villus sampling.

Blood Tests

Blood tests for pregnancy-associated plasma protein A (PAPP-A) and human chorionic gonadotropin (hCG) are done in the first trimester. The Papp-A is a test that screens for Down syndrome. The hCG test screens for the hormones of pregnancy itself, but unusual readings can indicate chromosomal abnormalities or other rare problems. Neither of these tests alone could indicate a problem pregnancy. However, an atypical reading on either might indicate a need for further testing.

Pregnant women are also tested for levels of AFP and estriol along with hGC. Atypical levels of these substances in the blood may mean that the fetus should be tested for Down syndrome, Edward's syndrome or other types of chromosome abnormality. However, there are other possible causes of abnormal readings, such as twins, or a miscalculation of the date of conception.

Sometimes a blood test is combined with an ultrasound scan in a First Trimester Screen, a non-invasive test used in early pregnancy.

Ultrasound

An ultrasound scan creates a sonogram, a picture of the fetus. It can help in diagnosis and treatment. Ultrasound can also be used to check for molar or etopic pregnancy.

In the first trimester, an NT test may be done with ultrasound. This test looks for nuchal translucency. Some fetuses with abnormalities show an oversized clear space on the back of their neck. This can mean congenital heart trouble, or possibly Down syndrome or similar disorders. The test only gives an indication, not a certain answer.

Ultrasound can also confirm the presence of twins, and generally check for fetal well-being. Later in pregnancy, ultrasound can show which way a baby is placed in the uterus, and how the uterus is preparing for birth.

Amniocentesis

This test is usually performed in the 16th to 18th week of pregnancy. A small amount of amniotic fluid is drawn out from the woman's uterus. The amniotic fluid is the liquid that the fetus floats in before birth. It contains bits of genetic material that have been shed into the fluid.

By testing this fluid, which contains some DNA from the fetus, doctors can determine if there may be certain genetic problems, such as Down syndrome. At the same time, the test will reveal gender, and it can also indicate how well the fetus' lungs are maturing.

Amniocentesis is often done using a hollow needle inserted through a woman's abdomen. It is very safe, but there is a slight risk of miscarriage. For a procedure done where amniocentesis is performed frequently, the risk of miscarriage may be about one in four hundred.

Chorionic Villus Sampling

This test is usually done in the 10th to 13th week of pregnancy. The doctor removes a tiny piece of the placenta, the tissue that connects the fetus to the mother. It is tested because it has the same DNA as the fetus. Because this test is invasive, it does carry a small risk of miscarriage.

Usually a small tube, a catheter, is inserted through the cervix, guided by ultrasound. A few cells are gently suctioned into the catheter. The cells are then examined.

CVS can look for chromosome abnormalities like Down syndrome, and similar disorders. It can also look for cystic fibrosis. It is one way to do a very accurate paternity test before birth.

CVS can be done earlier than amniocentesis, and test results are returned more quickly. It is a very accurate test, but it cannot test for neural tube defects like spina bifida. Perhaps 1% of CVS procedures result in miscarriage.

Percutaneous Umbilical Blood Sampling

This procedure is performed at 18 weeks or later. It is very similar to amniocentesis, except that the object is to gather a fetal blood sample rather than a sample of amniotic fluid. Guided by ultrasound, a needle is inserted into the umbilical cord and withdraws blood. This test

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