What Types of Genetic Tests are Performed During Pregnancy?

Pregnancy is an exciting new adventure with many unknowns. Some people work very hard to get to pregnancy, others were surprised to discover they were pregnant, while others may not be sure how they feel about their pregnancy. Regardless of the circumstances, when a person goes to that first prenatal appointment – typically in the first trimester – it can feel a little overwhelming with all the information a doctor or midwife may provide. Your healthcare provider will often discuss what is safe to eat or do in pregnancy, what to expect throughout your pregnancy, and what choices you have to make in pregnancy. Most importantly, everyone wishes for a healthy pregnancy and safe delivery!

It’s normal to not remember all the information that was given in the first prenatal appointment. As medical screening has advanced, there are now more options for genetic testing in pregnancy. Genetic counselors will often review genetic testing options with their patients and help them make the best decision for themselves and their pregnancy. Here is a summary of the options every pregnant woman has in pregnancy.

Nuchal Translucency Ultrasound: This ultrasound screening occurs between 11 and almost 14 weeks of pregnancy. An ultrasound technician will measure the thickness of the nuchal translucency of the baby, which is a pocket of fluid at the back of the baby’s neck. If the thickness of the nuchal translucency is within the correct range (typically below 3.0mm), the risk of chromosome abnormalities like Down syndrome are reduced. In addition, the ultrasound technician looks for the presence of the nasal bone. Often pregnancies affected with Down syndrome at this stage of pregnancy do not have the nasal bone present.

First Trimester Screening (bloodwork): At 10-14 weeks of pregnancy, you can have blood work (in addition to the nuchal translucency ultrasound), which also screens for chromosome abnormalities like Down syndrome. For women under the age of 35, it’s common to have the option to do the Sequential Screen, which is a two-part blood test (one blood draw in the first trimester and one blood draw in the second) that analyzes proteins in your blood that come off from the placenta. By analyzing these proteins, the screen will determine if the pregnancy is at high risk or low risk for Down syndrome, Trisomy 18, and neural tube defects. Women over the age of the 35 are often referred to genetic counseling to discuss their age-related risk of chromosome abnormalities and discuss screening options. Non-invasive Prenatal Testing (NIPT) is a blood test (performed any time after 10 weeks of pregnancy) that analyzes fetal DNA that comes off from the placenta and circulates in the mother’s blood. This test determines if the pregnancy is at high or low risk for Down syndrome, Trisomy 13, Trisomy 18, Turner syndrome, and Triploidy. This test is available to everyone, but health insurance doesn’t often cover the cost of the test for women under the age of 35. The accuracy of the NIPT blood test is higher than other screening methods.

Chorionic Villus Sampling (CVS): This is a diagnostic procedure performed between 10 and 13 weeks of pregnancy. A physician will determine where the placenta is located and inserts a needle through the abdomen or inserts a flexible catheter through the vagina to take a sample of the placenta for genetic testing. This procedure allows us to look at all of the chromosomes and analyze them for duplications, deletions, or pieces of chromosomes in the wrong place. Due to the invasive nature of the procedure, there is a risk of miscarriage which is approximately 1/200. Most people who are pregnant will not pursue diagnostic testing unless an abnormality is seen on ultrasound, they are over the age of 35 and do not wish to do screening, or they have a family history of a genetic condition.

Amniocentesis: This is another diagnostic procedure that is performed after 15 weeks of pregnancy. This procedure involves inserting a needle through the abdomen and withdrawing a few tablespoons of amniotic fluid that the fetus is floating around in. This procedure is performed via ultrasound guidance to avoid touching the fetus. By analyzing the amniotic fluid, we can look directly at the fetus’s chromosomes to determine if they are normal. This diagnostic procedure also has a risk of miscarriage which is between 1/500 to 1/700. Similar to the CVS procedure, most people who are pregnant will not pursue diagnostic testing unless there is an abnormality seen on ultrasound or they have a family history of a genetic condition.

Alpha Fetoprotein (AFP) Blood Test: This blood test can be performed after 15 weeks of pregnancy. The AFP levels are analyzed in the mother’s blood to determine if the pregnancy is at increased risk for neural tube defects. If the AFP levels are high, the mother is often sent to have an ultrasound to determine if the pregnancy is affected by a neural tube defect.

Fetal Anatomy Ultrasound: The ultrasound performed at 20 weeks of pregnancy will analyze all of the different parts of the fetus to determine if there are any major birth defects. In addition, the ultrasound will also determine if there are any signs of chromosome abnormalities like Down syndrome, Trisomy 13, or Trisomy 18.

Carrier Screening: This blood test can be performed before and during pregnancy. By analyzing your blood, the test looks at the patient’s DNA to determine if they are a carrier of any genetic conditions. The types of genetic conditions included in carrier screening require both parents to be carriers of a genetic condition in order for there to be a 1 in 4 or 25% chance of a pregnancy being affected. Typically, a carrier of a genetic condition is healthy, as one copy of their gene functions and the other does not. If a pregnancy inherits both non-functioning copies of the gene, one from each parent, then the pregnancy would have the genetic condition. Carrier screening can be done for just a few common genetic conditions but can also cover 250+ genetic conditions, depending on the test. In addition, women are screened for genetic conditions that only a mother can pass on to a son, which are called X-linked genetic conditions.

If you would like to discuss your options for genetic testing in pregnancy, have had an abnormal fetal ultrasound, or have a genetic condition running in your family, please contact AT-GC to schedule an appointment with a genetic counselor.

Resources:
https://www.acog.org/womens-health/faqs/prenatal-genetic-screening-tests

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