A neural tube defect (NTD) is a birth defect that occurs when the neural tube, a structure that becomes the brain and spinal cord, fails to close properly during early pregnancy. This happens during the third and fourth weeks of pregnancy, often before a woman even knows she’s pregnant. NTDs affect roughly 3 out of every 10,000 live births in the United States, and they range from mild conditions that go unnoticed for years to severe defects that are fatal at birth.
How the Neural Tube Forms
Around the third week after fertilization, a flat sheet of cells along the embryo’s back begins to fold inward, forming a narrow channel. This channel gradually zips closed from the middle outward in both directions. The upper portion becomes the baby’s brain and skull, while the lower portion becomes the spinal cord and the bones of the spine. The entire process takes roughly one week to complete.
When something disrupts this closure, the type of defect depends on where and when the failure occurs. A problem at the top of the neural tube affects brain development. A problem lower down affects the spine. The location and size of the opening largely determine how severe the condition will be.
Types of Neural Tube Defects
Anencephaly
Anencephaly is the most severe NTD. It occurs when the upper end of the neural tube fails to close, resulting in a baby born without major parts of the brain and skull. The forebrain and cerebrum, the regions responsible for thinking and coordination, do not develop. Almost all babies born with anencephaly die shortly after birth.
Spina Bifida
Spina bifida occurs when the lower part of the neural tube doesn’t close completely. It comes in three forms, varying widely in severity.
- Myelomeningocele is the most serious form. A sac of fluid pushes through an opening in the baby’s back, and part of the spinal cord and nerves sit inside that sac, exposed and damaged. This causes moderate to severe disabilities, including inability to move the legs.
- Meningocele involves a fluid sac protruding through the back, but the spinal cord itself stays in place. Because the nerves are largely spared, this form typically causes only minor disabilities.
- Spina bifida occulta is the mildest form. There’s a small gap in the spine, but no visible opening or fluid sac on the back. The spinal cord and nerves are usually normal. Many people with this condition don’t find out until late childhood or adulthood, often through an X-ray taken for an unrelated reason. It’s sometimes called “hidden” spina bifida.
Encephalocele
Encephalocele is a rarer defect where brain tissue and its surrounding membranes push through an opening in the skull, forming a sac-like bulge. The outlook depends on the size and location of the protrusion and how much brain tissue is involved.
Complications of Severe Spina Bifida
Children born with myelomeningocele often face additional challenges beyond the spinal defect itself. About 90% develop a condition called Chiari II malformation, where part of the brain is pushed downward into the spinal canal. This structural shift disrupts the normal flow of cerebrospinal fluid and frequently leads to hydrocephalus, a dangerous buildup of fluid in the brain. Many children with hydrocephalus need a surgically placed shunt to drain the excess fluid, and that shunt often requires replacement surgeries over time.
Depending on the location of the spinal opening, children may also experience bladder and bowel control problems, reduced sensation in the legs, and orthopedic issues like clubfoot or hip dislocations.
What Causes Neural Tube Defects
Most NTDs result from a combination of genetic and environmental factors rather than a single cause. The best-understood risk factor is insufficient folate (vitamin B9) during the weeks surrounding conception. Folate plays a critical role in cell division and DNA synthesis, both of which are happening at extraordinary speed during neural tube closure. Without enough of it, the rapidly dividing cells of the neural tube are more prone to errors.
Other factors that increase risk include poorly controlled diabetes before and during early pregnancy, obesity, high body temperature in the first trimester (from fever or hot tub use), and certain anti-seizure medications. A family history of NTDs also raises the odds significantly. Women who have had one NTD-affected pregnancy face a much higher risk in subsequent pregnancies.
How NTDs Are Detected During Pregnancy
Most NTDs are identified before birth through routine prenatal screening. Between weeks 15 and 20 of pregnancy, a blood test measures levels of a protein called alpha-fetoprotein (AFP). When the baby has an open NTD, this protein leaks from the exposed tissue into the amniotic fluid and then into the mother’s bloodstream, producing an abnormally high reading. An elevated AFP result doesn’t confirm a diagnosis on its own but prompts further testing.
A detailed ultrasound can often visualize the defect directly, showing the opening in the spine or skull. In some cases, amniocentesis (drawing a small amount of amniotic fluid) provides additional information. Anencephaly is typically visible on ultrasound by the second trimester.
Treatment and Long-Term Outlook
Treatment depends entirely on the type and severity of the defect. Spina bifida occulta usually requires no treatment at all. Meningocele is typically repaired surgically after birth with good outcomes.
Myelomeningocele has traditionally been repaired within the first day or two after birth to close the opening and protect the spinal cord from further damage. But a landmark study funded by the National Institutes of Health found that performing the surgery before birth produces significantly better results. Children who had prenatal repair walked independently 93% of the time, compared to 80% of those who had the traditional postnatal surgery. They also scored higher on motor skills assessments. Perhaps most strikingly, only 49% of the prenatal surgery group needed a shunt for hydrocephalus, compared to 85% in the postnatal group. These benefits persisted through school age.
Even with successful surgery, many children with myelomeningocele need ongoing support: physical therapy for mobility, management of bladder and bowel function, and monitoring for hydrocephalus. The level of independence varies widely depending on where on the spine the defect occurred.
Prevention With Folic Acid
Taking folic acid before and during early pregnancy is the single most effective way to reduce the risk of NTDs. The CDC recommends that all women capable of becoming pregnant take 400 micrograms (mcg) of folic acid daily, not just those actively planning a pregnancy. This matters because the neural tube closes so early, often before a missed period, that waiting until a positive pregnancy test is too late.
Women who have previously had a pregnancy affected by an NTD are advised to take a much higher dose: 4,000 mcg daily, starting at least one month before conception and continuing through the first three months of pregnancy. This tenfold increase reflects the substantially higher recurrence risk in these pregnancies.
Folic acid is found naturally in leafy greens, beans, and citrus fruits, and it’s added to enriched grain products like bread and cereal in the United States. But diet alone rarely provides a consistent 400 mcg per day, which is why a daily supplement or prenatal vitamin is the more reliable approach.