Making Sense Of The Diagnostic Tests During Pregnancy
Although both screening and testing for genetic disorders are offered to all pregnant women during the course of her prenatal care, there are women who are considered to be at a higher risk for having a baby with a genetic disorder. Many decisions regarding screenings are based on her family history.
Screening Vs. Testing
There is a difference between screening for genetic disorders and testing for genetic disorders.
The purpose of screening is to determine which individuals may have a higher risk factor for the disease or disorder.
Screening does not determine a diagnosis. It is simply a method to identify those who might have a specific condition from those who most likely do not. Screenings are not always accurate. The results may show that there is an increased chance of something being abnormal, when it isn’t. The results may also indicate that there is nothing abnormal, when there actually is. These types of errors are referred to as “false positive” and “false negative” results. 1 The results of a screening will be used to help you and your physician decide on the next step. The American College of Obstetricians and Gynecologists (ACOG) recommends that all pregnant women are offered screening, as well as testing, even if there are no risk factors identified. 2
Based on the results of a screening, your physician will discuss the options for additional testing. Testing, unlike screening, will determine of the condition being tested for is present or not.
Screenings For Genetic Disorders
Nuchal Translucency Ultrasound Screening For Down Syndrome
The Nuchal translucency ultrasound is performed between the 10th and 12th week of gestation. An ultrasound is done in the nuchal fold (a space that is filled with fluid located behind the baby’s neck). A nuchal fold that is greater than 3 millimeters means that there is an increased risk of Down syndrome. An increase in thickness may also indicate the presence of other genetic disorders such as heart defects and diaphragmatic hernia.
Nuchal Translucency Ultrasound https://fetalmedicine.org/var/uploads/File/0/0/N/T/_/NT_2k14_high.jpg
Maternal Serum Alpha Fetoprotein
A maternal serum alpha-fetoprotein screening involves collecting a sample of a pregnant woman’s blood. This is done during the 15th and 18th week of pregnancy. It is a screening tool for the possibility of a baby having a genetic disorder such as spina bifida or Down syndrome.
This screening is recommended to be performed between the 10th and 14th week of pregnancy. A blood sample is taken, and the level (amount) of PAPP-A (pregnancy-associated plasma protein A) and hCG (human chorionic gonadotropin) present in the bloodstream is measured. It is then compared to the woman’s age, weight and gestational age (how far along she is in pregnancy). A lower PAPP-A level and a higher hCG level could indicate a higher risk for Down syndrome.
Integrated (Sequential) Second Trimester Screening: Screening For Down Syndrome (Trisomy 21), Trisomy 18 And Neural Tube Defects
Integrated (also known as Sequential) screening includes two blood tests and an ultrasound. The first blood test is recommended to be drawn between the 11th and 13th week of pregnancy. The first blood test involves PAPP-A screening. For the second blood test, a quad screen is performed.
The second blood test is recommended to be drawn between week 15 and week 18 of pregnancy, but it may be performed up to week 21 of pregnancy.
Diagnostic Tests For Genetic Disorders
Chorionic villus sampling
Chorionic villus sampling is performed during the 10th and 14th week of pregnancy. During this procedure, the physician collects a small amount of tissue from the placenta, either through the cervix (transcervically) or through the abdomen (transabdominally). The small amount of placental tissue is sent to the lab and tested for what is called aneuploidy (chromosome abnormalities). It is also possible to determine the sex of the baby. The placental tissue may also be tested for Tay-Sachs disease, Huntington disease, Hemophilia A and cystic fibrosis.
An amniocentesis is performed during the 16th and 18th week of pregnancy. During this procedure, your physician, or a Maternal Fetal Medicine Specialist (a doctor who specializes in high risk pregnancies), inserts a needle into the abdomen, using an ultrasound machine to make sure that the needle goes into the right spot. A small amount of amniotic fluid will be withdrawn. The amniotic fluid will be tested to determine whether or not a fetus has a number of different conditions.
- Bellver, J., Casanova, C., Garrido, N., Lara, C., Remohi, J., Pellicer, A., & Serra, V. (2013). Additive effect of factors related to assisted conception on the reduction of maternal serum pregnancy-associated plasma protein A concentrations and the increased false-positive rates in first-trimester Down syndrome screening. Fertility and Sterility, 100(5), 1314-1320.e3. doi: 10.1016/j.fertnstert.2013.06.045
- Wilson, K., Czerwinski, J., Hoskovec, J., Noblin, S., Sullivan, C, Harbison, A., Campion, M., Devary, K., Devers, P., Singletary, C. (2012). NSGC practice guideline: prenatal screening and diagnostic testing options for chromosome aneuploidy. Journal of Genetic Counseling, 22(1), 4-15. doi: 10.1007/s10897-012-9545-3