The quad screen is a blood test typically performed between 15 and 20 weeks of pregnancy to detect women whose pregnancies may be at higher risk for three types of problems: open neural tube defects (such as spina bifida, an opening in the spine); Down syndrome and trisomy 18 (chromosome abnormalities).
The quad screen measures the levels of four chemicals in the mother's blood. One of these is alpha fetoprotein (AFP), a protein made in the baby's liver and secreted into the amniotic fluid surrounding the baby. All four chemicals cross the placenta and enter the mother's bloodstream, where they can be measured. Openings along the baby's body, such as along the spine or in the abdominal wall, allow more than normal amounts of AFP to leak from the baby and enter the amniotic fluid, increasing the level of AFP in the mother's bloodstream.
Causes of elevated AFP are:
Neural Tube Defects: Neural tube defects include both spina bifida (improper closure of the fetal spine) and anencephaly (improper closure of the fetal skull). Anencephaly is a lethal condition. Individuals with spina bifida can have variable problems as a result of the birth defect. These problems can include leg paralysis, lack of bladder and bowel control, hydrocephalus (increased fluid in the brain) and scoliosis (curved spine). The severity of impairment depends on the size and location of the opening - larger openings and those higher on the spine predict a more severe outcome.
Abdominal Wall Defects: Abdominal wall defects may result in the herniation of abdominal organs into a sac (omphalocele) or into the amniotic cavity (gastroschisis). An opening in the abdominal wall can often be surgically repaired at birth, with a good outcome.
Incorrect Dating: A pregnancy further along than believed at the time of the test would make the AFP appear elevated. Also, twin and triplet pregnancies create more AFP, increasing the blood levels and making the test results appear abnormal. Ultrasound can rule out multiple pregnancy and incorrect dates as causes of an elevated AFP.
Fetal Kidney Problems: Abnormalities of the fetal kidneys and of the urinary tract can be associated with highly elevated AFP levels.
Placental Problems: An elevated AFP not explained by the above possibilities may be due to a problem with the placenta. A poorly functioning placenta can cause pregnancy complications, such as miscarriage, delayed growth of the baby, early delivery, and pre-eclampsia (high blood pressure during pregnancy).
A False Positive Result: A final possibility is that the baby has no health problems. Some babies who are healthy produce more AFP than average. This is often the cause for false positive AFP tests.
Two procedures available during pregnancy may help identify problems associated with elevated AFP levels.
Ultrasound (sonogram) is a tool used to date the pregnancy and look carefully at the bones and internal organs of the baby. Ultrasound can also help detect openings along the spine, skull, and abdomen, and other birth defects. A detailed ultrasound at 18-20 weeks of pregnancy can detect up to 90% of babies with neural tube defects. Ultrasound cannot rule out all birth defects.
In addition to detecting chromosome abnormalities, amniocentesis can provide a measure of the AFP level in amniotic fluid. This technique allows for detection of greater than 98% of babies with open neural tube defects. Pregnancies in which an identifiable birth defect is not found may be more closely followed with ultrasound to monitor for complications associated with a poorly functioning placenta.