The quad screen is a simple blood test that screens pregnancies for Down syndrome, trisomy 18 and neural tube defects. It calculates the risk that your baby will have one of these disorders based upon the levels of four hormones in your blood, specifically alpha-fetoprotein, human chorionic gonadotropin, unconjugated estriol and inhibin. This screen is performed during the second trimester of pregnancy, between 15 and 22 weeks.
Down syndrome is a genetic condition caused when a baby receives extra genetic material from the egg or the sperm at the time of conception. In the case of Down syndrome, the extra material is always from an additional (or part of an additional) 21st chromosome. Babies with Down syndrome have some degree of mental retardation. Approximately one in 800 babies is born with Down syndrome.
Trisomy 18 is also a genetic condition, but the extra chromosome is a number 18. Most pregnancies affected by trisomy 18 are miscarried or result in stillbirth, but occasionally a child born with this condition can survive for several years. Babies with trisomy 18 usually have birth defects and all are severely mentally retarded. Approximately one in 5,000 babies is born with trisomy 18.
Neural tube defects are birth defects that affect a baby’s spine or brain, such as spina bifida or anencephaly. Spina bifida is an opening along the spine where the bones and skin don’t cover the spinal cord; the effects can range from leg weakness to quadriplegia. In anencephaly, the baby’s brain does not form completely, and babies born with this condition do not survive. About one in 1,000 babies is born with spina bifida, while about one in 2,000 babies is born with anencephaly.
If your quad screen is positive for an increased risk for one of these conditions, it is important to understand that this does not necessarily mean your baby is affected. For example, about 5 percent of quad screens will return positive for an increased risk of Down syndrome, but only about 2.5 percent of those pregnancies will be affected.
If you have a positive quad screen, we will refer you for genetic counseling to further discuss the results. You will then be offered an ultrasound, and possibly an amniocentesis, to positively confirm the presence or absence of these conditions. It is also important to understand that if the quad screen is negative, the possibility of Down syndrome, trisomy 18 and neural tube defects has not been eliminated. A negative screen only means that the chance of these three birth defects is very low.
This screen is optional. It can provide information about the health of your baby with no risks. If the results are positive and the follow-up tests confirm a birth defect, you will have time to discuss all of your options. Some patients might decide to terminate their pregnancies, but for those who don’t, this information can help determine the most appropriate care and guide delivery planning. But if you feel you would never end your pregnancy and do not want to know before birth if your baby has Down syndrome, trisomy 18 or a neural tube defect, you may choose not to have this screen.
If you have any questions about the quad screen, please don’t hesitate to ask them at your next prenatal visit.