What Causes Spina Bifida?

Spina bifida is a birth defect that happens when the neural tube — the structure that becomes the brain and spinal cord — does not close completely during the first month of pregnancy. The cause is not a single gene or a single exposure. It is the result of several factors interacting, and in most cases the exact trigger is never identified. The single best-established contributor is low folate (vitamin B9) around the time of conception.

What Causes Spina Bifida?

Spina bifida is a neural tube defect. The neural tube forms in the embryo and normally closes by about the 28th day after conception — often before a woman knows she is pregnant. When the lower end of the tube fails to close, the bones of the spine (vertebrae) do not form completely over the spinal cord. In the mildest form, this gap causes no symptoms. In the most serious form, the spinal cord and its coverings push through the opening.

No single cause has been found. Researchers describe it as multifactorial, meaning multiple genetic and environmental factors combine to raise risk. Folate deficiency is the factor with the strongest evidence behind it.

Why folate matters

Folate is a B vitamin the body uses to build DNA and divide cells. Rapid cell division is exactly what happens during neural tube formation. When folate is low at that precise window, the tube is more likely to close improperly.

The link is not theoretical. Randomized controlled trials — the strongest type of evidence — showed that folic acid supplements taken before and during early pregnancy reduce the occurrence of neural tube defects. That finding is why folic acid fortification of enriched grain products became standard in the United States in the late 1990s, and why neural tube defect rates dropped afterward.

This does not mean folate is the only cause. Many women with low folate have unaffected pregnancies, and many with normal folate have a child with spina bifida. Folate is a major modifiable factor, not the whole story.

How Genetics and Family History Affect Risk

Genetics clearly play a role, though the inheritance pattern is complex. Spina bifida does not follow a simple dominant or recessive pattern. Instead, many genes each contribute a small amount of risk.

The clearest signal comes from recurrence. A woman who has had one child with a neural tube defect has a higher chance of having another. This is why higher-dose folic acid is often recommended for subsequent pregnancies — a practice supported by clinical evidence, though the exact dose is a medical decision made with a clinician.

Certain genetic conditions raise risk more sharply. Chromosomal abnormalities, including trisomy 18 and trisomy 13, are associated with neural tube defects. So are some rare single-gene disorders. For most people, though, there is no identifiable genetic syndrome — just a background of inherited susceptibility.

Which Medical Conditions and Medications Raise Risk?

Several maternal conditions are linked to higher rates of spina bifida.

• Diabetes — poorly controlled blood sugar before and during early pregnancy is a well-documented risk factor.
• Obesity — higher pre-pregnancy body mass index is associated with increased risk, though the reason is not fully understood.
• Hyperthermia — prolonged high fever in early pregnancy, or exposure to high heat such as a sauna, has been linked to neural tube defects.

Medication is another consideration. Valproic acid and carbamazepine, both used to treat seizures and some mood disorders, are associated with increased neural tube defect risk. This does not mean these drugs are unsafe for everyone — for some people they are the only effective option. It means the decision needs to be made with a clinician, ideally before pregnancy, so alternatives and folate supplementation can be discussed.

It is worth separating two things here. A risk factor raises the odds of a problem. It does not cause the problem in any given pregnancy. Most women with diabetes, or who take valproic acid, have babies without neural tube defects.

Does Folic Acid Actually Prevent Spina Bifida?

Yes, for a meaningful share of cases. This is one of the better-supported findings in preventive medicine.

The evidence comes from randomized trials and from population data after fortification. When folic acid intake around conception goes up, neural tube defect rates go down. The effect is strongest for folic acid taken before conception and through the first weeks of pregnancy — the window when the neural tube closes.

Timing is the part people miss. Starting folic acid after a positive pregnancy test is often too late for neural tube closure, because the tube has already formed by then. This is why the recommendation is aimed at anyone who could become pregnant, not just those who are trying.

Standard guidance for most women is 400 micrograms of folic acid daily before and during early pregnancy. Women with a prior affected pregnancy, or who take certain medications, are often advised to take a higher dose under medical supervision. Folate from food also counts, but folic acid from supplements and fortified foods is better absorbed.

What Role Do Environmental Exposures Play?

Environmental factors have been studied, and some associations exist, but the evidence is weaker than for folate or genetics.

Maternal hyperthermia — from fever or heat exposure — is one of the more consistent environmental findings. Some studies suggest certain agricultural chemicals or solvents may be linked, but results vary and no single exposure has been confirmed as a cause.

It is easy to overstate this area. When people read that “environmental factors” contribute, they often assume a specific toxin is responsible. For most cases, no such culprit is found. The honest position is that environmental contributions are real but poorly defined, and they likely matter less than folate status and genetics.

Can Spina Bifida Be Prevented?

Not in every case, but risk can be reduced. The most evidence-backed step is adequate folic acid before conception.

Beyond that, managing blood sugar in diabetes before pregnancy, avoiding prolonged high heat and treating fever promptly, and reviewing medications with a clinician before conception all address known risk factors. None of these guarantees an unaffected pregnancy, and none of them is a reason for blame if spina bifida occurs.

This last point matters. Spina bifida is not caused by something a mother did wrong. It arises from a combination of factors, many of which are outside anyone’s control. Prevention advice is about lowering odds, not assigning fault.

How Is Spina Bifida Detected During Pregnancy?

Spina bifida can often be detected before birth, which allows planning and, in some cases, treatment.

Blood tests in the second trimester can show elevated levels of alpha-fetoprotein (AFP), a protein linked to open neural tube defects. Ultrasound can sometimes show the opening directly, along with related signs in the brain. In some cases, amniocentesis — testing fluid around the fetus — provides more information.

Detection is not the same as prevention. Finding spina bifida prenatally does not change whether it happened. It changes what options are available, including the possibility of fetal surgery in select cases, which some research indicates can reduce the need for a shunt later in life. That surgery carries its own risks and is not appropriate for every case.

What Does Not Cause Spina Bifida

Several claims circulate that are not supported by evidence. Spina bifida is not caused by a mother’s diet in general, by stress, by exercise, or by a single food. It is not an inherited condition in the simple sense that it runs predictably through families. And it is not caused by vaccines — no credible evidence links vaccination to neural tube defects.

Clearing these up is not just about accuracy. When people believe a false cause, they may focus on the wrong prevention steps or feel unnecessary guilt.