Angela D. Earhart, M.D.
Moncenya Chatman, M.D.
Perinatal Medical Group

Questions and Answers


Why should I have prenatal screening?

According to the American College of Obstetricians and Gynecologists (ACOG), almost all children in the United States are born healthy. Only 2 or 3 out of 100 newborns have major birth defects. For the majority of babies the cause is unknown. However, there are certain birth defects that can be tested for prenatally-before a baby is born. These include Down syndrome, trisomy 18, trisomy 13, and open neural tube defects. The risk of having a baby with a chromosome abnormality, such as Down syndrome, increases with the mother's age. However, ACOG recommends prenatal testing be offered to all pregnant women, regardless of age.

What is Down syndrome?

Down syndrome, also known as trisomy 21, is the most commonly occurring chromosome abnormality. It is caused by having an extra copy of chromosome 21-for a total of 3 copies instead of the normal 2. One in every 733 babies is born with Down syndrome. There are more than 400,000 people living with Down syndrome in the United States.

What is trisomy 18?

Trisomy 18, or Edwards syndrome, IS the second most common trisomy-a condition in which someone has 3 copies of a certain chromosome. It is caused by having an extra copy of chromosome 18-for a total of 3 copies instead of the normal 2. About half of the babies with trisomy 18 who reach full term are stillborn. Trisomy 18 causes developmental problems associated with life-threatening complications in a baby's first months and years. These may include deformities of the heart, intestines, esophagus, hands, and feet; kidney problems; delayed growth; and mental retardation.

What is trisomy 13?

Trisomy 13, or Patau syndrome, is another well-known chromosome abnormality. It is caused by having an extra copy of chromosome l3-for a total of 3 copies instead of the normal 2. Only 5% to 10% of children with trisomy 13 live past their first year. Trisomy 13 is associated with severe mental and physical problems that cause many infants to die during their first days or weeks of life. Infants with trisomy 13 often have problems including abnormalities of the heart, brain, or spinal cord; small or poorly developed eyes; extra fingers and/or toes; deformed mouths; and weak muscle tone.

What are open neural tube defects?

Open neural tube defects, or ONTDs, are another type of birth defect that occurs very early in a baby's development before most women even know they are pregnant. The risk of having a baby with an ONTD does not increase with the age of the mother. One in every 1,000 babies is born with an ONTD. Spina bifida is an example of an ONTD. It is caused when the fetus' spine fails to close properly during development. Spina bifida causes major damage to the nerves and spinal cord and other physical problems. Paralysis of the lower limbs and learning disabilities may also result.

Why is prenatal screening important?

Being tested during pregnancy will allow you to be informed about the health of your baby, even before it is born. The information you receive will be extremely valuable in helping you plan for your baby's birth. You and your doctor will also be able to decide if you need more testing later in your pregnancy. If you learn you will have a baby with one of these birth defects, you can proactively prepare your family for the future by making important medical and financial decisions about care.

How is prenatal screening performed?

Part 1: First Trimester Screening

Part 1 consists of 2 components. The first is a simple blood test performed on you when a fetus is between 9 weeks and 13 weeks, 6 days of age. Your blood is analyzed for 2 markers normally

found in all pregnant women. The blood test is followed by an ultrasound examination, given when the fetus is between 11 weeks, 1 day and 13 weeks, 6 days of age. The ultrasound confirms your baby's age and measures the amount of fluid behind the baby's neck. Results of the blood test and the ultrasound are combined to estimate the risk of Down syndrome, trisomy 18, and trisomy 13. 85 to 90% of Down syndrome cases are detected. Approximately 75% of trisomy 18 and trisomy 13 cases are detected.

Part 2: Second Trimester Screening

Second trimester screening comprises the Quad Screen, which is performed in the second trimester of your pregnancy, between 15 weeks and 21 weeks, 6 days. This part of the test analyzes 4 more markers found in the blood of all pregnant women. 94% of Down syndrome cases are detected when Part 1 and Part 2 results are combined. 95% of trisomy 18 and trisomy 13 cases are detected when Part 1 and Part 2 results are combined. Also screening for ONTDs, may be performed, which can only be done in the second trimester

What do the results of each part of the screening tests mean?

Part 1: First Trimester Screen

Negative screen

A negative screen occurs in about 95% of all tests. This means your baby is at low risk for Down syndrome, trisomy 18, or trisomy 13. If you get a negative screen, your doctor may decide to stop screening or may continue with the Quad Screen in your second trimester for additional verification.

Positive screen

A positive screen occurs in about 5% of all tests. This means your baby is at increased risk for Down syndrome, trisomy 18, or trisomy 13. When you receive this result in your first trimester, you and your doctor may choose to consider additional diagnostic testing options, like chorionic villus sampling (CVS) or amniocentesis.

Part 2: Quad Screen

Negative screen

A negative screen in your second trimester confirms your baby is at low risk for Down syndrome, trisomy 18, trisomy 13, or ONTDs. However, it does not completely eliminate the possibility your baby may have Down syndrome, trisomy 18, trisomy 13, ONTDs, or any other chromosome abnormalities not detectable through a screening test.

Positive screen

A positive screen means your baby is at increased risk for Down syndrome, trisomy 18, trisomy 13, or ONTDs.

Your doctor may recommend a diagnostic testing option, such as amniocentesis.

If I receive a positive screen, what additional tests may I take?

If you are at risk of having a baby with a birth defect based on your family history or the results of a previous screening test, your doctor may recommend diagnostic testing. There are 2 options you can consider. One test, known as chorionic villus sampling, or CVS, is performed at 10 to 12 weeks of pregnancy. A small tube is inserted into your cervix, or a thin needle is inserted through your cervix or abdomen in order to remove a small piece of the placenta The sample is sent to a lab and the cells are checked for chromosome abnormalities such as Down syndrome, trisomy 18, and trisomy 13. CVS does not test for ONTDs. CVS is associated with a small risk of miscarriage. Another test, known as amniocentesis, is performed between 16 and 20 weeks of pregnancy. A small sample of fluid surrounding the baby is taken through a needle and sent

to a lab to be tested. The cells in the fluid are checked for chromosome abnormalities such as Down syndrome, trisomy 18, and trisomy 13. Amniocentesis also tests for ONTDs. Amniocentesis is associated with a small risk of miscarriage. Be sure to discuss these additional tests with your doctor to determine which options may be right for you.