This test is used to identify pregnancies that may be at higher risk for Down syndrome, spina bifida, and Trisomy 18. The spine problem (spina bifida) occurs when the fetus (baby in your womb) doesn’t form a complete covering around the major parts of the nervous system (brain and spinal cord). It may lead to various physical problems ranging from needing braces to walk to severe handicaps including, not being able to walk as well as mental problems, and even death.
Down syndrome means the baby has an extra chromosome 21. Down syndrome causes mental retardation, heart abnormalities, and other birth defects. Trisomy 18 is due to an extra 18 chromosome and causes mental retardation and other abnormalities more severe than Down syndrome.
The decision to have the test done is left up to you. In this country, the overall chance of having a child born with a spine defect is about one to two out of every 1,000 babies born. However, if you, the baby’s father, or any relatives have had a child with spina bifida, the chance goes up. Even though there is an increased risk of having a spine defect because of a family history, most of the children with spine defects are born to couples without anyone in the family having had it. If you have ever had a child with anencephaly (where the brain and scalp do not develop normally), this is considered a type of spinal defect and should be reported to your doctor. The chances of having a child with Down Syndrome and Trisomy 18 are about one in 800 for all women, but chances are higher for women who are over 35 years of age. In general, as you get older, your chances of having a child with a chromosome problem increases.
An abnormal screen does not necessarily mean that the fetus has a problem, but it means your risk is higher. An abnormal risk may mean there is incorrect dating of your pregnancy or you may be carrying more than one fetus. Your doctor may suggest a more detailed ultrasound test to look at the fetus, or more testing may be offered to make sure the fetus doesn’t have Down syndrome, spina bifida, or other chromosome problems. After you have read this material, you should discuss any questions with your doctor or medical staff.
Your doctor will explain the possible meanings of the abnormal test and order some further tests, which may include ultrasonography and/or an amniocentesis, to help discover the cause of the abnormal test. The decision of having these other tests done will be left up to you. If you decide to have further testing done, your doctor may do the testing or send you to a specialist.
No test is completely foolproof. Rarely, this test can be normal even though you may be carrying a fetus with a spine defect or Down syndrome.
Fortunately, this test is very good. Just by itself, it will detect 80% of all spine defects and about 60% to 70% of fetuses with Down syndrome. However, a normal result on this test does not guarantee a normal baby or a baby free of these or other birth defects.