ONTD stands for open neural tube defect, a group of birth defects where the brain, spinal cord, or spine doesn’t form properly during early pregnancy. These defects happen when the neural tube, a structure that eventually becomes your baby’s brain and spinal cord, fails to close completely. The term most often comes up during second-trimester screening, when a blood test can flag a higher chance of ONTD.
How the Neural Tube Forms
The neural tube is one of the first major structures to develop in an embryo. It starts as a flat sheet of cells that folds inward and zips shut, first in the middle, then toward each end. The top end closes around day 24 after conception, and the bottom end closes around day 26. By four weeks, the tube is normally sealed completely, forming a closed system that will become the brain and spinal cord.
When part of the tube doesn’t close, the brain or spinal cord is left exposed. Because this happens so early, most neural tube defects form before many people even realize they’re pregnant.
Types of Open Neural Tube Defects
There are three main types of ONTD, depending on where the neural tube fails to close:
- Spina bifida. The most common type. The spine doesn’t form properly over the spinal cord, leaving part of the cord exposed or protruding through a gap in the vertebrae. The severity varies widely. In the mildest form, there may be no symptoms at all. In the most serious form, called myelomeningocele, the spinal cord and its protective membranes push outward through an opening in the back, which can cause paralysis, bladder and bowel problems, and fluid buildup in the brain.
- Anencephaly. The neural tube fails to close at the top, so major portions of the brain and skull don’t develop. This is not compatible with life. Babies with anencephaly are either stillborn or survive only hours to days after birth.
- Encephalocele. Brain tissue or its coverings push through an opening in the skull. This can occur anywhere from the forehead to the back of the head, or occasionally near the nose and sinuses. Outcomes depend on how much brain tissue is involved.
How ONTD Is Detected During Pregnancy
ONTD is typically screened for between weeks 15 and 20 of pregnancy using a blood test that measures a protein called alpha-fetoprotein (AFP). Your baby naturally produces AFP, and small amounts cross into your bloodstream. When there’s an open defect, more AFP than usual leaks into the amniotic fluid and then into your blood, producing a higher-than-normal reading.
A high AFP result doesn’t mean your baby has an ONTD. It’s a screening test, not a diagnosis. Elevated levels can also result from carrying twins, being further along than estimated, or other factors. If your AFP comes back high, your provider will typically follow up with a detailed ultrasound to look directly at the baby’s spine and skull. Amniocentesis, where a small sample of amniotic fluid is tested, can provide additional confirmation.
Risk Factors
The single biggest modifiable risk factor is low folate (vitamin B9) levels during early pregnancy. Beyond that, the CDC identifies several other factors that raise the likelihood of a neural tube defect:
- Poorly controlled diabetes. Pre-existing diabetes that isn’t well managed before and during early pregnancy increases risk.
- Certain medications. Some anti-seizure drugs interfere with how the body uses folic acid, which can prevent the neural tube from closing normally.
- Overheating. Fever or prolonged exposure to high heat, such as hot tub use, during the first weeks of pregnancy has been linked to higher risk.
- Previous affected pregnancy. Having one baby with a neural tube defect raises the chance of it happening again in a future pregnancy.
Prevention With Folic Acid
Taking folic acid before and during early pregnancy is the most effective way to reduce the risk of ONTD. The U.S. Preventive Services Task Force recommends that anyone planning to or who could become pregnant take 400 to 800 micrograms of folic acid daily. This is the dose found in most prenatal vitamins.
Because the neural tube closes by day 26, well before a first missed period, the supplement needs to be on board before conception to be effective. Starting folic acid after a positive pregnancy test is often too late to prevent a neural tube defect that was going to occur.
People at higher risk, including those with a previously affected pregnancy, a family history of neural tube defects, or those taking medications that block folic acid, typically need a higher dose prescribed by their provider.
Treatment Options for Spina Bifida
Anencephaly and severe encephalocele have limited treatment options, but spina bifida, the most common ONTD, can sometimes be treated before the baby is born. Fetal surgery to repair the spinal opening while the baby is still in the uterus became a recognized treatment option after a landmark clinical trial (known as the MOMS study) showed it could reduce the need for a shunt to drain fluid from the brain and improve motor outcomes compared to surgery after birth.
Not every pregnancy qualifies for fetal surgery. Eligibility depends on specific criteria: the defect needs to fall within a certain range of the spine, the baby must have a normal chromosomal makeup, and referral needs to happen between 19 and 26 weeks of pregnancy. Pregnancies with twins, certain placenta problems, or a maternal BMI above 35 are generally excluded. The surgery carries its own risks, including preterm delivery, so the decision involves carefully weighing potential benefits against those risks.
When fetal surgery isn’t an option or isn’t chosen, the opening is surgically closed within the first day or two after birth. Long-term care for a child with spina bifida depends on where the defect is located on the spine and how much nerve function is affected. Children with lower defects tend to have better mobility, while higher defects can involve more significant paralysis and require ongoing support for bladder and bowel management, mobility, and in some cases, treatment for fluid buildup in the brain.