What is genetic screening?

If you are concerned about illnesses in your family that a child might inherit, you can ask your health care provider for genetic counseling or screening. Genetic screening is a way to learn more about the diseases or birth defects a child might inherit from his or her parents.

It is best to do genetic screening before you are pregnant. However, it may also be done at your first prenatal visit and later in your pregnancy.

At your genetic screening visit, your provider or a genetic counselor will ask about any diseases, disorders, or birth defects in your family. Samples of your blood might be tested. If you are pregnant, tests to examine the baby's condition and chromosomes may be done.

When is it used?

Some of the most common reasons for a couple to have genetic screening, given in order of most common to least common, include:

• The mother will be 35 years old or older at the time of delivery.

• They have had a child with an inherited disease or birth defect.

• Someone in the family has had mental retardation or another chromosome problems.

• The couple has had stillbirths (babies born dead after 28 weeks of pregnancy) or several miscarriages.

• The health care provider has found an abnormality, such as abnormal blood test results or too much or too little amniotic fluid around the baby.

• Someone in the family has had thalassemia, a disorder causing abnormal red blood cells. Thalassemia is most common in people from Southeast Asia, China, and Mediterranean countries (for example, Italians and Greeks).

• Someone in the family has had Tay-Sachs disease, a brain disorder that can cause early death. This disease is most common in Jews who have an Eastern European Ashkenazic ancestry.

• Family members have had other inherited disorders, such as hemophilia (a blood clotting disorder) or cystic fibrosis.

• Someone in the family has had sickle cell anemia, another disorder that causes abnormal red blood cells. This disorder is most common in North American blacks.

How do I prepare for genetic screening?

Learn the medical history of people in your family. Try to get details of any inherited diseases in your family. Ask your parents and your spouse's parents if there have been any retarded, disabled, or abnormal children in their families.

Also be prepared to give the following information about yourself:

• any past miscarriages

• any exposure to chemicals, radiation (including x-rays), or other environmental hazards (for example, at work or from hobbies) before or during pregnancy

• any history of drug or alcohol abuse

• any prescription or nonprescription drugs taken by the mother since she became pregnant, including before she knew she was pregnant.

How is genetic screening done?

Your health care provider or genetic counselor will review your family and personal medical histories. In addition, the following tests are used to look for birth defects and inherited diseases when you are pregnant:

• Maternal blood tests, including a check of the level of alpha fetoprotein (AFP). AFP is a protein made by the fetus. If the amount of AFP is high or low, your health care provider may do other tests. These tests look for defects in the nervous system called neural tube defects, such as spina bifida, or for chromosome defects.

• Ultrasound scans to check the baby for birth defects of the brain, heart, spine, legs, arms, or other organs.

• Chorionic villus sampling to test a sample of tissue from the placenta for chromosomal abnormalities.

• Amniocentesis to test the amniotic fluid around the baby for abnormal chromosomes and other substances such as alpha fetoprotein.

Your health care provider or counsellor will discuss the results with you. If there is a problem, they will help you understand the problem and describe your choices for prevention or treatment.