Spina bifida is one of a class of birth defects, called neural tube defects. These defects involve damage to the spine and spinal cord.

Screening and diagnosis

If you're pregnant, you'll be offered prenatal screening tests to check for spina bifida and other birth defects. These tests may provide reassurance that your fetus doesn't have certain serious 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 test (MSAFP). To perform this test, your doctor draws a blood sample to be sent 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 may indicate 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.

Results are most accurate when the test is performed between 16 and 18 weeks of gestation, but spina bifida usually can't be diagnosed by blood test alone. Up to 20 percent of spina bifida cases don't produce a high level of AFP. In addition, when a high level of AFP is found, a neural tube defect is present only 10 percent 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 diagnostic tests, such as ultrasound and amniocentesis.

Your doctor may pair 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 or quadruple screen. In addition to checking for neural tube defects, looking at these additional substances can help doctors determine whether chromosome abnormalities are present.

Ultrasound

If blood tests indicate high AFP levels, your doctor may 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 small 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 raise 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. Ultrasound performed in a perinatal center will detect more than 95 percent of babies with spina bifida when high AFP levels have increased a doctor's suspicions. Ultrasound is safe for both mother and baby.

Amniocentesis

If a blood test shows high levels of AFP in your blood, your doctor may recommend amniocentesis in addition to ultrasound. During amniocentesis, your doctor removes samples 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. Although amniocentesis can't indicate the severity of spina bifida, it's likely that a fetus has a neural tube defect when there are high levels of AFP in the amniotic fluid and all other causes have been ruled out. This test does have a risk to the developing fetus. About one in 200 pregnant women miscarries after amniocentesis.