Screening for and Diagnosing Chromosomal Abnormalities

Maternal Blood Screening

The pregnant person's blood will be sampled at 15 to 18 weeks of pregnancy. It will be tested for substances that can point to a chromosomal problem in the baby. It does not mean that there is a problem. It only suggests that there is a higher risk.

If the levels are abnormal, more tests may be done to get more information. The pregnant person may need an ultrasound and amniocentesis.

AFP (Alpha-Feto Protein) Screen

The AFP screen is the most common test for birth defects. The fetus’ liver and yolk sac make AFP. It can enter the pregnant person's blood during pregnancy. This blood test is done at 16 to 18 weeks of pregnancy.

Higher than normal AFP levels point to a neural tube defect. This is a problem with how bones form around the spinal cord. AFP leaks out of the spinal column and into the pregnant person's blood. This causes higher levels of AFP than normal. A neural tube defect can lead to spina bifida and problems with brain growth.

A lower than normal AFP level can point to Down syndrome or another problem with the chromosomes.

This screening test is common, but it is not exact. There is a high rate of false results. This means where that the test does not show a problem when one may exist. If the levels are abnormal, other tests may be given. These tests can give you more detail and a better diagnosis. The next steps may be an ultrasound and amniocentesis.

Triple Screen

The triple screen tests the pregnant person's blood between 15 and 17 weeks of pregnancy. It is done to look for:

• Alpha-fetoprotein (AFP)
• Human chorionic gonadotropin (hCG)—a hormone made in the placenta
• Estriol—an estrogen made by the fetus and the placenta

These substances are all normally found in certain amounts. If the levels are too high or too low, it may point to a problem.

This test may be needed in pregnant people who:

• Have a family history of birth defects
• Are 35 years of age or older
• Used harmful medicines or drugs during pregnancy
• Have diabetes and use insulin
• Had a viral infection during pregnancy
• Have been exposed to high levels of radiation

Quad Screen

The quad screen looks for four substances in the pregnant person's blood:

• Alpha-fetoprotein (AFP)
• Human chorionic gonadotropin (hCG)
• Estriol
• Inhibin-A—a protein made by the placenta and ovaries

This test is like the triple screen. But the quad test may be more exact for Down syndrome. The quad test also has a lower rate of missing problems.

Ultrasound

Ultrasound is used to make pictures of the fetus. It is often done 10 to 14 weeks into a pregnancy. The doctor will look for the neck fold of the baby. The fold is larger than normal in a baby with Down syndrome or Turner syndrome.

Amniocentesis

This test removes a small amount of fluid that surrounds the baby. It is done at 15 to 18 weeks of pregnancy. Substances in the fluid may point to certain genetic defects.

The doctor will put a thin needle into the belly. An ultrasound will help to guide the needle to the right spot. A small amount of the fluid is drawn out. There is a small risk of miscarriage.

This test may be done on pregnant people who:

• Are over 35 years of age
• Have a prior child with a birth defect that can be diagnosed by amniocentesis
• Have a family history of a genetic problem
• Have an abnormal triple screen test result

Chorionic Villus Sampling (CVS)

CVS offers more genetic detail than amniocentesis. It can be done earlier in pregnancy and gives quicker results.

CVS is a more invasive procedure. It is done between the 10th and 12th week of pregnancy. The doctor removes a tiny sample from the inside of the uterus. The sample comes from small areas that surround the fetus in the early weeks. This area later forms the placenta. The sample may be taken by putting a thin tube through the vagina and cervix. It may also be done by putting a needle through the abdomen.

There is a small risk of miscarriage, but it is higher than the risk for amniocentesis. Vaginal bleeding is also common.

This test may be done on pregnant people who:

• Are over 35 years of age
• Have a child with a genetic defect
• Have a family history of a genetic disorder
• Have an abnormal first trimester screening