NTD stands for neural tube defect, a group of birth defects that occur when the neural tube, the structure that becomes a baby’s brain and spinal cord, doesn’t close completely during early pregnancy. This closure happens around the sixth week of pregnancy (four weeks after conception), often before many people even know they’re pregnant. The two most common NTDs are spina bifida and anencephaly, and taking folic acid before and during early pregnancy can prevent the majority of cases.
How the Neural Tube Forms
In the earliest weeks of pregnancy, a flat strip of cells along the embryo’s back folds inward and seals itself into a tube. This tube eventually becomes the entire central nervous system: the brain at one end and the spinal cord running down the back. If the tube fails to close at the top, near the skull, the result is a brain defect. If it fails to close somewhere along the spine, the result is a spinal defect. Because this process wraps up so early, the window for something to go wrong is narrow but critical.
Types of Neural Tube Defects
Spina Bifida
Spina bifida occurs when the neural tube doesn’t close along the spine. It ranges widely in severity:
- Spina bifida occulta is the mildest and most common form. A small gap exists in one or more vertebrae, but the spinal nerves aren’t involved. Most people with this type have no symptoms at all. The only visible sign may be a tuft of hair, a small dimple, or a birthmark on the skin over the gap.
- Meningocele is rare. A sac of spinal fluid bulges through an opening in the spine, but the spinal cord and nerves stay in place. Children with this type may have some bladder or bowel difficulties, but neurological function is largely preserved.
- Myelomeningocele is the most serious form, also called open spina bifida. The spinal canal is open along several vertebrae, and a sac containing exposed spinal nerves and tissue protrudes from the baby’s back at birth. This type can cause loss of movement in the legs along with bladder and bowel dysfunction.
Anencephaly
Anencephaly occurs when the neural tube fails to close at the base of the skull. The brain develops without part or all of the cerebrum, the region responsible for thinking, vision, hearing, and movement. Bone is missing over the back of the head and sometimes around the front and sides as well. A baby born with anencephaly may have basic reflexes but cannot survive without the cerebrum. There is no treatment, and care focuses on supporting families through the loss.
Encephalocele
Encephalocele is much rarer. In this defect, brain tissue or its coverings push through an opening in the skull. Outcomes depend on the size and location of the protrusion and how much brain tissue is involved.
Risk Factors
The CDC identifies several factors that raise the likelihood of an NTD-affected pregnancy:
- Low folate levels during early pregnancy are the single most well-established risk factor.
- Pre-existing diabetes that isn’t well controlled before conception.
- Certain anti-seizure medications taken during early pregnancy.
- Overheating from hot tub use or high fever during the first trimester.
Having a previous pregnancy affected by an NTD also places you in a higher-risk category for future pregnancies.
How NTDs Are Detected
Most NTDs are caught during routine prenatal screening, typically through two methods. A blood test measuring alpha-fetoprotein (AFP), a protein made by the developing baby, is drawn between 16 and 22 weeks of pregnancy. When AFP levels are higher than expected, it can signal an open NTD, though elevated levels don’t always mean a defect is present.
Ultrasound is the more definitive tool. In studies of pregnancies with elevated AFP, ultrasound alone correctly identified 98% of neural tube defects with 100% specificity, meaning it almost never flags a defect that isn’t there. The mid-pregnancy anatomy scan, usually performed around 18 to 22 weeks, is the standard point at which these defects are visually identified. Anencephaly can often be spotted even earlier because the missing skull bones are visible on imaging.
Prevention With Folic Acid
Folic acid (vitamin B9) is the most effective way to reduce NTD risk. The CDC recommends that all women capable of becoming pregnant take 400 micrograms (mcg) of folic acid every day, whether or not they’re planning a pregnancy. Because the neural tube closes before most people have their first prenatal visit, waiting until a positive pregnancy test is often too late. If you are actively planning to conceive, start taking folic acid at least one month before conception and continue through the first three months of pregnancy.
For people at higher risk, specifically those who have had a previous NTD-affected pregnancy, who have diabetes, or who take anti-seizure medication, the recommended dose jumps significantly. Clinical trial data showed that 4,000 mcg (4 mg) of folic acid per day substantially reduced recurrence of NTDs compared to placebo. In the UK, guidance recommends 5 mg daily for high-risk individuals, a figure based on clinical consensus rather than trial evidence but considered safe given no known harm from higher doses. Your provider can help determine which dose applies to you.
Fortified foods like enriched breads, cereals, and pastas also contribute to your daily folate intake, but a supplement is the most reliable way to hit the target. When choosing a vitamin, look for one that lists 400 mcg of folic acid on the label. More is not necessarily better for standard-risk pregnancies.
Treatment for Spina Bifida
Anencephaly and severe encephalocele have no surgical correction, but spina bifida, particularly myelomeningocele, has real treatment options. The exposed spinal tissue can be surgically closed after birth to protect the nerves and reduce infection risk. The more groundbreaking option is fetal surgery, where the defect is repaired while the baby is still in the womb.
A landmark clinical trial known as the MOMS trial compared prenatal repair (performed between 19 and 26 weeks of pregnancy) with standard postnatal surgery. The results were striking. Only 40% of babies who had fetal surgery needed a shunt to drain excess fluid from the brain by age one, compared to 82% in the postnatal surgery group. At 30 months, 42% of the fetal surgery group were walking independently versus 21% of those repaired after birth. The surgery also significantly reversed the downward herniation of the brain into the spinal canal, a complication common in myelomeningocele.
Fetal surgery is not available to everyone. Candidates must meet specific criteria: the pregnancy must be a singleton, the lesion must be at a certain spinal level, and the mother cannot have conditions like uncontrolled diabetes or a BMI of 35 or higher. The procedure also carries risks, including preterm delivery, so it involves careful evaluation at a specialized center. For babies who don’t qualify for prenatal repair, postnatal surgery remains effective at protecting remaining nerve function.
Children born with myelomeningocele typically need ongoing care from multiple specialists to manage mobility, bladder and bowel function, and related complications. With modern treatment, most children with spina bifida survive into adulthood, though the level of independence varies depending on where on the spine the defect occurred and how much nerve function was preserved.