Prenatal Genetic Testing

Our bodies are made up of tiny cells. Chromosomes are the packages of genetic information (DNA) inside our cells. They contain the instructions for our growth and development. Usually, each person has a total of 46 chromosomes in each cell. The chromosomes come in pairs (23 pairs). Each parent contributes one chromosome to each pair.

When a fetus (developing baby) has extra or missing chromosomes, it can lead to medical problems. An extra chromosome (three copies instead of two) is called a trisomy. A missing chromosome (one copy instead of two) is called a monosomy. This type of genetic condition doesn’t usually run in families. It can happen just by chance in any pregnancy.

A screening test estimates the chances of certain chromosome abnormalities in pregnancy. These tests are not invasive and pose no risk to the fetus. The results are based on your age and other factors.

Screening tests can identify pregnancies that have an increased chance of having a chromosome abnormality, but they don’t tell whether one is actually present. If you decide to have screening and your result suggests an increased chance of an abnormality, we’ll give you details on other tests that may provide more information. If your results do not suggest an increased chance for an abnormality, you may not need extra testing.

Your doctor or genetic counselor will explain more about what your results may mean. Screening tests have a risk of false results. That is, some people who have a pregnancy with a chromosome abnormality will be told their pregnancy is low-risk. Others who have a pregnancy with no chromosome abnormality will be told the pregnancy is high-risk. The chance of these false results depends on the test performed.

Four main types of screening tests for chromosome abnormalities are available.

Early Risk Assessment

Early risk assessment (ERA) uses blood work and ultrasound. For this screening, the ultrasound looks at the nuchal translucency. This refers to fluid behind the neck of a fetus. It is normal for fluid to be present in the first trimester. The area of fluid is often larger in fetuses with chromosome abnormalities and certain other problems, including heart problems.

To do an ERA, the testing team draws blood between 9 and 13 weeks’ gestation. We perform the ultrasound between 11.5 and 13 weeks’ gestation. This test detects about 85–90% of fetuses with Down syndrome and about 90% of fetuses with trisomy 13 or trisomy 18.

After ERA screening, you may wish to have a second blood test at 15 to 21 weeks’ gestation. This is sequential screening. This increases the chance of detecting Down syndrome and trisomy 18 to more than 92%. The results rely on information from the first and second blood tests and the first trimester ultrasound.

Sequential screening is thought to be most useful when the ERA results indicate an intermediate chance of an abnormality. If the ERA screen shows a very low chance for an abnormality, repeating the screen at 15 to 21 weeks is not likely to show different results. Those whose ERA results indicate a high chance for an abnormality might choose diagnostic testing or cell-free DNA (cfDNA) testing.

Quadruple Screen

This is a blood test taken between 15 and 21 weeks of pregnancy. We offer this screen to those who chose not to or weren’t able to have ERA. This test detects about 75–80% of fetuses with Down syndrome and about 65% of fetuses with trisomy 18.

Cell-Free DNA Analysis (cfDNA)

Cell-free DNA analysis (cfDNA) is a blood test. It looks at small pieces of DNA from the pregnancy that are in the blood. We can test these pieces to estimate the chance for Down syndrome, trisomy 18, trisomy 13 and sex chromosome abnormalities. Another name for this test is noninvasive prenatal testing (NIPT).

This test is available to all patients, including those with an increased risk of having a pregnancy affected with a chromosome abnormality (due to age 35 or older, or other factors). In high-risk individuals, this test can identify up to 99% of affected pregnancies. In some cases, insurance will not cover this test for those who aren’t at increased risk.

Fetal Survey

We perform this ultrasound during the second trimester. Another name for this test is a fetal anatomy survey. It uses sound waves to get pictures of the growing fetus. The purpose of this ultrasound is to see how the fetus is growing and to see if there are any noticeable physical problems.

About half (50%) of fetuses with Down syndrome will have some sign on ultrasound. When we see certain abnormalities, they increase the chance of a chromosome abnormality. If we see a physical problem, we consider many factors to decide how high the increased chance is for you. These factors include your age, any previous blood screening results and the type of abnormality seen on ultrasound.

About 5% of those who have ERA, sequential screening or quadruple screening will hear their pregnancy has an increased chance of having a chromosome abnormality. This doesn’t mean that a baby has a chromosome abnormality. In fact, for most pregnancies with this type of screening result, additional tests are normal, and the baby is healthy.

The actual chance of an abnormality depends on many factors and is specific to each pregnancy. If your screening test indicates an increased chance, your doctor and/or genetic counselor will explain what the results mean for you.

We’ll tell you about any extra testing that we can do. This includes cfDNA or diagnostic testing. If you have cfDNA testing that indicates an increased chance for a chromosome abnormality, we offer you diagnostic testing.

A diagnostic test can tell whether a chromosome abnormality is actually present. Anyone who is pregnant can choose to have a diagnostic test, regardless of the risk of chromosomal abnormalities. These tests count the chromosomes and look for any chromosomal number differences, including those that are less common.

The two types of diagnostic tests are chorionic villus sampling (CVS) and amniocentesis. Both types involve taking some tissue or fluid from the uterus (womb). Because of this, there is a very small chance that these tests could cause a miscarriage.

Chorionic Villus Sampling (CVS)

Chorionic villus sampling (CVS) involves taking small pieces of the placenta at 10 to 13 weeks of pregnancy. To obtain the small pieces of placenta, we insert a needle into the pregnant abdomen or a catheter through the cervix. The doctor uses ultrasound to locate a place to take the sample. The chance of having a miscarriage caused by amniocentesis is estimated to be about 1 in 400 (0.3%).

Amniocentesis

We can perform amniocentesis after 15 weeks of pregnancy. This test involves taking a sample of the fluid surrounding the fetus. To obtain the fluid, we insert a needle into the pregnant abdomen. The doctor uses ultrasound to locate a place to take the sample. The chance of having a miscarriage caused by amniocentesis is estimated to be about 1 in 900 (0.1%).