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Tests and diagnosis
By Mayo Clinic staff
If you're pregnant, you'll be offered prenatal screening tests to check for spina bifida and other birth defects. The tests aren't perfect. Even if the results are negative, there's still a small chance that spina bifida is present, and most mothers who have positive blood tests have normal babies. Talk to your doctor about prenatal testing, its risks and how you might handle the results. Prenatal testing is a personal choice.
Blood tests
The primary test used to check for myelomeningocele is the maternal serum alpha-fetoprotein (MSAFP) test. To perform this test, your doctor draws a blood sample and sends it to a laboratory, where it's tested for alpha-fetoprotein (AFP) — a protein that's produced by the fetus. It's normal for a small amount of AFP to cross the placenta and enter the mother's bloodstream, but abnormally high levels of AFP suggest that the fetus has a neural tube defect, most commonly spina bifida or anencephaly, a condition characterized by an underdeveloped brain and an incomplete skull.
Some spina bifida cases don't produce a high level of AFP. On the other hand, when a high level of AFP is found, a neural tube defect is present only a small percentage of the time. Varying levels of AFP can be caused by other factors — including a miscalculation in fetal age or multiple fetuses — so your doctor may order a follow-up blood test for confirmation. If the results are still high, you'll need further evaluation, including an ultrasound examination.
Your doctor may perform the MSAFP test with two or three other blood tests, which look for:
- Human chorionic gonadotropin (HCG), a hormone produced in the placenta
- Inhibin A, another hormone produced in the placenta
- Estriol, an estrogen produced by both the fetus and the placenta
Depending on the number of tests, the combination is called a triple screen or quadruple screen (quad screen). These tests are commonly done with the MSAFP test, but their objective is to screen for trisomy 21 (Down syndrome), not neural tube defects.
Ultrasound
Many obstetricians rely on ultrasonography to screen for spina bifida. If blood tests indicate high AFP levels, your doctor will suggest an ultrasound exam to help determine why. The most common ultrasound exams bounce high-frequency sound waves off tissues in your body to form black-and-white images on a video monitor.
The information these images provide can help establish whether there's more than one fetus and can help confirm gestational age — two factors that can affect AFP levels. An advanced ultrasound can also detect signs of spina bifida, such as an open spine or particular features in your baby's brain that indicate spina bifida.
In expert hands, ultrasound today is quite effective in detecting spina bifida and assessing its severity. Ultrasound is safe for both mother and baby.
Amniocentesis
If a blood test shows high levels of AFP in your blood but the ultrasound is normal, your doctor may offer amniocentesis. During amniocentesis, your doctor uses a needle to remove a sample of fluid from the amniotic sac that surrounds the fetus. An analysis indicates the level of AFP present in the amniotic fluid.
A small amount of AFP is normally found in amniotic fluid. However, when an open neural tube defect is present, the amniotic fluid contains an elevated amount of AFP because the skin surrounding the baby's spine is gone and AFP leaks into the amniotic sac. A second test can be done on the same sample to reliably confirm that a neural tube defect is present.
Discuss the risks of this test, including a slight risk of loss of the pregnancy, with your doctor.
- Spina bifida fact sheet. National Institute of Neurological Disorders. http://www.ninds.nih.gov/disorders/spina_bifida/detail_spina_bifida.htm. Accessed July 30, 2009.
- Spina bifida. The Merck Manuals: The Merck Manual for Healthcare Professionals. http://www.merck.com/mmpe/print/sec19/ch292/ch292c.html. Accessed July 30, 2009.
- Spina bifida, March of Dimes. http://www.marchofdimes.com/pnhec/4439_1224.asp. Accessed July 30, 2009.
- Hochberg L. Prenatal screening and diagnosis of neural tube defects. http://www.uptodate.com/index/home.html. Accessed July 30, 2009.
- Ask the doctor archive. Spina Bifida Association. http://www.spinabifidaassociation.org/site/c.liKWL7PLLrF/b.2738091/k.9A71/Risk_Factor.htm. Accessed Aug. 1, 2009.
- Cameron M, et al. Prenatal screening and diagnosis of neural tube defects. Prenatal Diagnosis. 2009;29:402.
- Routine tests in pregnancy. American College of Obstetricians and Gynecologists. http://www.acog.org/publications/patient_education/bp133.cfm. Accessed Aug. 1, 2009.
- FAQs about folic acid. Centers for Disease Control and Prevention. http://www.cdc.gov/ncbddd/folicacid/faqs.html. Accessed July 30, 2009.
- Edwards MJ. Hyperthermia and fever during pregnancy. Birth Defects Research (Part A): Clinical and Molecular Teratology. 2006;76:507.
- Folic acid for the prevention of neural tube defects: U.S. Preventive Services Task Force recommendation statement. Rockville, Md.: U.S. Preventive Services Task Force. http://www.annals.org/cgi/reprint/150/9/626.pdf. Accessed Aug. 24, 2009.
- Harms RW (expert opinion). Mayo Clinic, Rochester, Minn. Aug. 24, 2009.
