What Causes Neural Tube Defects in the Fetus?

Medically Reviewed on 1/26/2023
Causes of Neural Tube Defects
Neural tube defects are typically diagnosed in the fetal stage.

The exact causes of neural tube defects are not known. Multiple factors, including a combination of genetics, nutrition, and environmental factors, can cause it.

Evidence suggests that neural tube defects are more common among infants whose mothers have the following conditions:

  • Are obese
  • Have poorly controlled diabetes
  • Have folate deficiency
  • Are very young at the time of conception
  • Take certain antiseizure medications, including Dilantin (phenytoin), Tegretol (carbamazepine), Depakote (valproic acid), or antifolate (such as aminopterin)
  • Used a hot tub or sauna during the first four to six weeks of pregnancy
  • Have a history of the previous child born with a neural tube defect
  • Have a partner (biological father) with an immediate family member born with a neural tube defect

What are neural tube defects?

Neural tube defects (also called spinal dysraphism) are developmental neurological disorders present at birth due to malformations of the spine, spinal cord, and brain.

During early embryonic development, the developing baby grows a flat primitive tissue structure called a neural tube that later rolls together into a complicated structure and becomes the baby’s spinal cord, spine, brain, and skull.

Neural tube defects occur very early in pregnancy (about a month after conception) due to improper development or closure of the neural tube.

An estimated risk ratio for a pregnancy with a neural tube defect is less than one in 1,000.

What are the types of neural tube defects?

The three most common types of neural tube defects include the following:

  1. Spina bifida: A spinal cord defect results in an abnormal formation (sac-like protrusion along the backbone) of the back bones, spinal cord, and nerves, leading to a portion of the spinal cord and surrounding structure developing outside the body.
    • Meningomyelocele: Characterized by a sac of fluid protruding through an opening in the baby’s back, without the involvement or damage of the spinal cord.
    • Myelomeningocele (open spina bifida): The most severe form of spina bifida. Characterized by an incomplete neural tube closure and a fluid-filled sac protruding from the baby’s back, containing a part of the spinal cord, meninges, nerves, and cerebrospinal fluid.
    • Spina bifida occulta: The mildest form of spina bifida and characterized by a small gap in the baby’s spine, with no opening or sac on the back and no damage to the nerves and spinal cord.
  2. Encephalocele: A hole in the skull causes a sac-like protrusion along the head or near the nose during development. This causes brain tissues, fluids, and covering membranes to develop outside the skull.
  3. Anencephaly:
    • When the neural tube does not close at the base of the skull, fetuses are typically missing their forebrain, cerebrum, and bone or skin covering the remaining parts of their brain.
    • Fetuses with anencephaly often do not survive after 20 weeks of pregnancy or live very long after birth.

What are the symptoms of neural tube defects?

The symptoms of neural tube defects vary depending on the condition, severity, and the affected region.

  • Symptoms of spina bifida
    • A small hairy patch, dimple, birthmark, or sac on the back
    • Decreased or no sensation below the sac area
    • Legs paralysis
    • Chronic constipation
    • Hydrocephalus (accumulation of fluid, causing increased pressure in the head)
    • Heart problems
    • Bone issues
    • Intellectual disability
  • Symptoms of encephalocele
    • Presence of a sac on the skull
    • Facial defects
    • Hydrocephalus
    • Smaller head size
    • Weak arms or legs
    • Trouble walking
    • Vision problems
    • Intellectual disability
    • Delayed growth and development
    • Seizures
  • Symptoms of anencephaly
    • Absence of bones on the back of the head
    • Missing bones on the front and/or lateral side of the head
    • Missing large areas of the brain
    • Folded ears
    • Cleft palate
    • Congenital heart defects


Conception: The Amazing Journey from Egg to Embryo See Slideshow

How are neural tube defects diagnosed?

Neural tube defects are often diagnosed in the fetal stage (womb). In some rare cases, they are recognized until after birth.

Common tests done during the pregnancy to diagnose a neural tube defect include:

  • Prenatal ultrasound: Sound waves are used to create images to view the baby’s internal organs and overall development.
  • Anomaly scan: An ultrasound focusing on identifying physical problems at about 18 to 20 weeks of gestation.
  • Fetal MRI: Detailed pictures of organs, bones, and tissues without using ionizing radiation (X-rays) can be performed as early as 20 weeks to detect fetal abnormalities.
  • Triple screen: Elevated levels of alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG), and estriol (uE3) mark the presence of any abnormality in the fetus.
  • Quad screen: Measures levels of AFP, HCG, uE3, and inhibin A in the mother’s blood.
  • Maternal serum alpha-fetoprotein: A sample of the mother’s blood is analyzed for the levels of AFP.
  • Amniocentesis: A sample of fluid from the amniotic sac is withdrawn and analyzed.

How are neural tube defects treated?

Specific treatment of neural tube defects depends on the type and severity of the condition.

  • Treatment of spina bifida
    • Surgery
    • Insertion of a shunt to help drain the fluid and release the pressure inside the cranium (for hydrocephalus)
    • Additional therapies (to help walk or manage the bowel and bladder)
    • Rehabilitation
    • Positioning aids and splints
  • Treatment of encephalocele
    • Surgery to close the opening.
  • Treatment of anencephaly
    • There is currently no standard treatment for this condition. Babies born with this condition often do not live for more than a few days or weeks after birth.

Can you prevent neural tube defects during pregnancy?

According to the U.S. Public Health Service recommendation, getting enough folic acid supplements (400 mcg/day) before conception and early in pregnancy (during the first three months) can greatly reduce the risk of spina bifida and other neural tube defects.

Folic acid can be obtained through folate-rich foods, such as leafy green vegetables, fruits, dried beans and legumes, and fortified bread.

Medically Reviewed on 1/26/2023
Image Source: iStock image

Neural Tube Defects. https://www.hopkinsmedicine.org/health/conditions-and-diseases/neural-tube-defects

Facts about Neural Tube Defects. https://www.cdc.gov/ncbddd/birthdefects/facts-about-neural-tube-defects.html

Neural Tube Defects (NTD). https://my.clevelandclinic.org/health/diseases/22656-neural-tube-defects-ntd