Genetic Screening

Genetic carrier screening is a type of genetic screening that aims to determine if an individual is a carrier of a gene for a specific disorder. Most often, carrier screenings are used to look for recessive disorders. With recessive disorders, an individual must inherit two genes -- one from their mother and one from their father -- in order to present with the associated disorder. When an individual has only one gene instead of both genes, they are known as carriers. Carriers typically do not present with the disorder or disease. However, depending on the disorder, some carriers may have mild symptoms. 

Carrier screening is used to spot altered genes related to a limited number of disorders and diseases. These disorders and diseases include cystic fibrosis, a disorder that affects breathing and digestion; fragile X syndrome, one of the most common causes of inherited intellectual disability; sickle cell disease, a condition of the blood that can cause recurrent pain, infection, and fatigue; and Tay-Sachs disease, which involves destruction of the nerve cells of the brain and spinal cord. 

Many of these genetic conditions occur more commonly in certain ethnic groups. For example, sickle cell disease occurs more frequently in individuals of African descent, while Tay-Sachs disease typically affects individuals of Eastern or Central European Jewish, Canadian, or Cajun descent. Such predispositions sometimes lead individuals with certain ethnic ancestry to undergo genetic screening before having children, so they can better understand the probability of a child being born with a specific disorder. Some women choose to complete carrier screening during pregnancy. 

Carrier screening tests require a sample of an individual’s blood, saliva, or tissue, usually from the inside of the cheek. The parent who is most likely to be a carrier is typically tested first. If this parent tests negative and is determined to not be a carrier, no further testing is required. If, however, they are determined to be a carrier, the other parent is also tested. 

Prenatal genetic screening is not completely accurate, and the rate of inaccuracy varies from test to test. In general, these noninvasive prenatal tests, which can involve blood tests, ultrasounds, and DNA testing, are usually conducted during the first or second trimester of pregnancy. If results indicate an increased risk of a specific genetic disorder, further genetic testing is typically conducted because screening test results alone cannot make a definitive diagnosis. 

The purpose of genetic screening is to determine which individuals are at a higher risk of developing a certain disease or disorder, while genetic testing, which is often conducted after genetic screening has indicated high risk for a condition, is used to determine if the individual has the condition or not. 

Genetic screening most commonly screens for Down syndrome, Edwards syndrome, and brain or neural tube defects. Down syndrome is a genetic disorder caused by a third copy of chromosome 21, and it is associated with intellectual disability and characteristic facial features. Edwards syndrome, a genetic disorder caused by a third copy of chromosome 18, is associated with a low birth weight, clenched hands, and an unusually shaped head. Finally, brain or neural tube defects can cause disorders like spina bifida, in which the fetal spinal cord does not close completely during development, and anencephaly, a disorder in which most of the brain and skull do not develop. 

In the first trimester, screening commonly includes a blood test to measure two substances in the blood, called beta-hCG and PAPP-A. Decreased levels of beta-hCG and PAPP-A in the blood of the mother are linked to an increased risk of Edwards syndrome. It is also common at this point to perform nuchal translucency screening, which is an ultrasound exam that measures the thickness of the space on the fetus’ neck. An atypical nuchal length is linked to an increased risk of Down syndrome and physical defects of the heart, abdominal wall, and skeleton. 

In the second trimester, a “quad screen” is completed to measure the levels of four different substances in the blood: AFP, beta-hCG, estriol, and Inhibin A. Additionally, another ultrasound is conducted to screen for major defects affecting the brain, spine, facial features, abdomen, heart, and limbs. 

Genetic screening is used to identify the risk an individual or group of individuals has for developing a certain disorder. One type of genetic screening is genetic carrier screening, which identifies individuals who may carry atypical genes associated with  genetic disorders. Generally, prospective parents undergo carrier screening during family planning to assess the risk of their child being born with certain genetic disorders, but carrier screening can also occur during pregnancy. Carrier screening is most commonly used to assess risk for recessive disorders, such as sickle cell disease and Tay-Sachs disease. While not 100% accurate, genetic screening during pregnancy, commonly consisting of blood tests and ultrasound imaging, is widely employed to detect a variety of diseases and disorders in the fetus, including Down syndrome, Edwards syndrome, and brain or neural tube defects.