A quad screen is a blood test performed on the mother typically in the second trimester of pregnancy, between week 15 and week 20. This test looks for four specific substances in the blood to rule out any potential genetic defects with the baby such as Down Syndrome. Down Syndrome is a genetic disorder that can cause mental retardation and development delays, among other problems. Down syndrome can range from moderate to severe, and cause lifelong challenges to the baby as well as family members.
The quad screen requires a blood test. The blood is analyzed in a lab for four specific substances, AFP, hCG, estriol and inhibin-A. It is the inhibin-A that increases the chances of identifying babies at a higher risk for Down syndrome.
Alpha-fetoprotein, or AFP, is a fetal protein. Human chorionic gonadotrophin, or hCG, is a hormone produced in the placenta. Estriol is an estrogen produced in the baby and the placenta, and inhibin-A is a protein produced in the placenta and in the ovaries.
The level of these different substances is then combined with the mother’s risk factors to determine the potential for a genetic disorder in the baby. An abnormal amount of hCG and estiol combined with a decreased level of AFP, for example, may indicate the baby is at risk for Down syndrome. An elevated level of AFP may mean the baby is at risk for developing a defect such as spina bifida or anencephaly.
Spina bifida is a disorder where the neural tube, or the spinal cord and its coverings, have not completely developed. On some occasions spina bifida can cause hydrocephalus, an accumulation of fluids on the brain. Anencephaly, on the other hand, is a disorder where the neural tube has not completely developed near the head. This can result in the absence of portions of the brain, skull or scalp.
Maternal risk factors to consider in addition to the quad screen include things like family history of birth defects and pregnancy after the age of 35. The use of drugs or medications may also have an impact as can a pre-existing condition such as diabetes. Even things like radiation exposure or a viral infection during the pregnancy can increase the risk factor.
The quad screen is not a diagnostic or identifying test. It simply screens the risk factors. This means that an abnormal quad screen may or may not result in a baby with a genetic disorder. If risk factors are high, further testing may be needed to assess the possibility of a genetic disorder. Ultrasound and amniocentesis are the standard course of action following an abnormal quad screen.