Prenatal Services in the Division of Genetics
The Division of Genetics provides screening tests for genetic disorders during or before pregnancy. All pregnant women need to be screened to determine whether your developing fetus is at risk for abnormalities. During the first trimester of pregnancy, the Division of Genetics offers a group screening and counseling program at Mount Sinai West (212 523-5895).
Depending on the results of your First Trimester Screen, results of an alternative screening test, or due to other indicators you and your partner may have, additional tests may be required or requested. In these cases, the Division of Genetics coordinates further testing for fetal abnormalities. This includes risk assessment and genetic counseling, so that you understand your results and choices throughout the screening and testing process.
The great majority of babies are born healthy. However, the average pregnancy has an approximate 3 or 4% risk of a birth defect, mental retardation or genetic disorder. This section of the Web site will introduce you to the tests available at Mount Sinai St. Luke’s and Mount Sinai West to determine the health of your developing fetus.
The information presented here is not a substitute for advice from your doctor or genetic counselor. However, reading it will prepare you to talk with your health care providers and make informed choices.
Most prenatal tests can be divided into 2 categories, screening and diagnostic. The results of a screening test will tell you how much risk there is of an abnormality in the pregnancy. Diagnostic tests actually confirm whether or not a fetal abnormality exists. In this section of the Web site, we will describe the most common screening and diagnostic tests and then outline some of your choices. The topics available are outlined below. Please click on the link to select an area of interest.
A screening test is a risk assessment. It determines the likelihood that certain abnormalities are present in a pregnancy. If your likelihood of having an abnormality is higher than a certain cutoff value, the screening-test results will be classified as positive. If your risk of an abnormality is lower than that cutoff value, the screening-test results will be classified as negative. When your results become available, you will be informed whether the results are positive or negative; you may also be informed of the numerical chances that an abnormality is present.
It is important to understand that a positive result does not indicate that an abnormality is present. Instead, a positive screen (and the level of risk associated with it) may prompt you to pursue further tests. Similarly, a negative screening result does not rule out an abnormality. Instead, for women younger than 35, a negative result means your risk is significantly lowered, and so you may decide not to have further testing.
Because the risk of fetal chromosome abnormalities increases with maternal age, all pregnant women age 35 and over should consider diagnostic testing (chorionic villus sampling or amniocentesis), even if screening results are negative.
For women younger than 35, a screening test is a good tool to help you decide whether you need a diagnostic test. Screening tests are generally simple, relatively inexpensive, and noninvasive.
Nuchal Translucency and First Trimester Screening
Nuchal translucency (NT) refers to an ultrasound measurement of the back of the fetal neck, which can be done from approximately 11 through 13 weeks of pregnancy. First-trimester screening combines the NT measurement with the results of a blood test, your age, and your ethnic background. From these factors, a personal risk estimate is calculated. A positive result will identify 85% of Down syndrome cases and 80% of Trisomy 18 cases. In other words, if your fetus has Down syndrome, there is an 85% chance that the test will identify it. Likewise, if your fetus has Trisomy 18, there is an 80% chance the test will be positive for Trisomy 18. This test is a screen; in other words, it does not tell you definitively whether there is a fetal abnormality. It simply determines the chances that your fetus has Down syndrome or Trisomy 18, and reveals whether those risks are high or low. The results are usually complete within 1 week. An abnormal first-trimester screen allows you to consider the option of the first-trimester diagnostic test, CVS (see later).
Second Trimester Screening: Maternal Serum Screen (Also referred to as Triple Screen or Quad Screen)
The maternal serum screen is the second-trimester alternative to first-trimester screening. It begins with a blood test performed between 15 and 18 weeks of pregnancy to determine the levels of 3 or 4 different pregnancy-related proteins. The protein levels are analyzed in combination with other factors, including the number of weeks you have been pregnant, your age, ethnic background, and weight. The results provide specific risk estimates for having a fetus with Down syndrome, Trisomy 18 and open neural tube defects like spina bifida. A positive result will detect each disorder in 65% to 70% of cases where it is present. The results are usually complete within 1 week of the blood test. An abnormal maternal serum screen allows you to consider the option of amniocentesis.
Integrated screening is a combination of the first- and second-trimester screening tests, and it is done in 2 stages. First, a nuchal translucency measurement and a blood test are performed between 11 and 13 weeks of pregnancy. These results are not released. Instead, between 15 and 18 weeks of pregnancy, the second-trimester serum screen is performed (see above). The information from the first- and second-trimester tests are then analyzed in combination, to provide you with specific risk assessments for Down syndrome, Trisomy 18 and open neural tube defects such as spina bifida. Among the screening tests, the integrated screen has the highest detection rate for Down syndrome; a positive result will identify approximately 94% of cases. The test also identifies an estimated 90% of Trisomy 18 cases and 80% of open neural tube defects. Results from integrated screening are provided in the second trimester and allow for the option of amniocentesis testing (see later). At Mount Sinai St. Luke’s and Mount Sinai West, integrated screening is coordinated by a genetic counselor, who will meet with you to review the benefits, risks and limitations of the test.
Anatomy Scan/ Level II Ultrasound
The anatomy scan is a detailed ultrasound recommended for all pregnant women between 18 and 20 weeks of pregnancy. Since every pregnancy faces an approximate 2 to 3% chance for birth defects, the anatomy scan provides a thorough examination of fetal structure and development. The doctor performing the scan will look at the structure of the fetal heart, brain, lungs, stomach, intestines, kidneys, and other major organs. A measurement of the fetal bones will also be obtained. Depending on the quality of the scan, maternal weight, and fetal positioning, small birth defects may be difficult to detect; however, the anatomy scan can detect many major birth defects and fetal growth problems. If you wish, the sex of the fetus can often be determined at this stage. It is important to understand that an anatomy scan does not detect chromosomal abnormalities.
Maternal age of 35 or above
Many of the abnormalities detected by prenatal tests fall into three categories: chromosomal abnormalities, open neural tube defects, or genetic disorders.
Normally, the fetal cells contain 23 pairs of chromosomes, or packages of genetic information (DNA). A missing or extra chromosome can cause an abnormality in the way a fetus develops. Down syndrome and Trisomy 18 are two common types of chromosomal abnormalities.
Open Neural Tube Defects
The neural tube forms early in pregnancy. If it fails to close properly, an opening can remain along the head or spine of the fetus. Spina bifida is the name for such an opening along the spine.
Genes are the individual units of heredity located on the 23 pairs of chromosomes. Human beings have an estimated 30,000 genes that contribute to fetal growth and development, including things like eye color, blood type, and intelligence. Just as chromosomes are inherited in pairs, genes are also inherited in pairs, one from each parent. Genetic disorders can occur when there is a change, or mutation, in a specific pair of genes. Your risk for genetic disorders depends on your family history and ethnic background
Certain genetic disorders (defined above) are more common in specific ethnic populations. For example, cystic fibrosis is more common in people of European Caucasian descent. In those of African and Mediterranean descent, sickle cell anemia, thalassemias, and other hemoglobin disorders are more abundant. In the Ashkenazi Jewish population, a number of disorders, including Tay-Sachs disease, Canavan disease and familial dysautonomia occur in higher frequency.
Your ethnicity and your partner's ethnicity will help determine if your current or intended pregnancy could be at risk for genetic disorders. If so, blood tests can be performed to check if you or your partner is a carrier of a genetic disorder. Sometimes it is best to have these blood tests before you become pregnant, but they can also be done in early pregnancy. Ask your provider which tests are right for you. If both partners are carriers for the same disorder, diagnostic testing of the fetus may also be warranted.
Diagnostic tests provide you with concrete information about the health of your pregnancy. While screening tests only provide an assessment of the possible risks, a diagnostic test will tell for sure if there is an abnormality in the fetal chromosomes and will clarify the exact nature of the problem. Also, if you have a family history of a specific genetic disorder, specialized DNA testing can be performed using diagnostic tests. Diagnostic testing is available to all women who are found to be at increased risk of carrying a pregnancy with a fetal anomaly. In addition, because the risk of fetal chromosome abnormalities increases with maternal age, all pregnant women age 35 and above should consider diagnostic testing, even if screening results are negative.
First Trimester Diagnosis: Chorionic Villus Sampling
Chorionic villus sampling (CVS) yields a definitive diagnosis of fetal chromosomal abnormalities and most genetic disorders. This test is performed between 10 and 12 weeks of pregnancy by a perinatologist. In the procedure, a small amount of the placenta is taken for analysis. The placental tissue may be obtained either vaginally or abdominally, depending on the position of the placenta in the uterus. Complications after CVS are estimated to occur in 1% of women. The main advantage of this test is that definitive results are available early in pregnancy, making further choices such as termination of pregnancy or corrective fetal surgery more practical. At Mount Sinai St. Luke’s and Mount Sinai West, CVS is coordinated by a genetic counselor, who will meet with you to review the benefits, risks and limitations of the test.
Second Trimester Diagnosis: Amniocentesis
Amniocentesis is typically performed between 16 and 18 weeks of pregnancy. Like CVS, it provides a definitive diagnosis of fetal chromosomal abnormalities and most genetic disorders. In amniocentesis, a sample of fluid is collected for analysis from the amniotic sac surrounding the fetus. Using ultrasound guidance, a needle is inserted through your abdomen to withdraw the fluid. Analysis of a protein in the fluid known as AFP provides additional information about the development of the fetal spine. Complications after amniocentesis occur in about 1 of every 200 procedures. Your doctor will offer an amniocentesis test if you are at increased risk or if you are 35 or older. At Mount Sinai St. Luke’s and Mount Sinai West, amniocentesis is coordinated by a genetic counselor, who will meet with you to review the benefits, risks and limitations of the test.
If you are younger than 35 and begin prenatal testing during the first trimester, you will usually be offered a choice between 2 screening tests, first-trimester screening or integrated screening. The screening test you choose will determine some of your other testing options (see flowchart below). There are limitations and benefits to each of these strategies.
If you choose to have first-trimester screening, you will receive your results early in pregnancy. If the screening results are positive you will have the option to receive diagnosis by CVS in the first trimester (see CVS, above).
If you choose to have integrated screening, you will not receive the screening results until the second trimester, and the only diagnostic test available will be amniocentesis (see above). However, integrated screening has benefits: it detects a higher percentage of fetal abnormalities; it also detects spinal bifida and other open neural tube defects.