What is Anencephaly?

Reviewed by Dan Brennan, MD on June 15, 2021

Anencephaly is a serious birth defect where a baby is born without parts of their skull and brain. It's a rare type of neural tube defect that affects about 1 in 4,600 babies. There is no way to treat anencephaly. Babies born with this condition will die before or shortly after birth. 

What Is Anencephaly?

Anencephaly is a type of neural tube defect. During pregnancy, your baby's brain and spine start to form as a flat disk of cells. This flat disk rolls into a tube-like structure called the neural tube. By 28 to 32 days after conception, the neural tube is completely formed. When part of the neural tube doesn't close completely, there's an opening called a neural tube defect. 

Sometimes this opening is exposed, or open, and sometimes it's covered with bone or skin. In anencephaly, this opening is at the base of the skull.  

What Causes Anencephaly?

Several things seem to cause anencephaly. Some of these are things you are exposed to when you are pregnant. It may also be related to your nutrition and genetics. If you've had one child with a neural tube defect, you are more likely to have a child with anencephaly. Some other factors that may increase your baby's risk of having it include: 

Not getting enough folic acid. If you don't get enough folic acid during your pregnancy, you have a higher chance of having a baby with anencephaly. You should take a prenatal vitamin that has at least 400 micrograms of folic acid before and during your pregnancy.

Certain medications. Some medications used to treat seizures, bipolar disorder, and migraines can cause neural tube defects. These include phenytoin, carbamazepine, and valproic acid

Genetic disorders. Anencephaly can be caused by some genetic disorders, such as trisomy 18. These disorders are usually not inherited. Instead, they happen by chance.

High temperature during pregnancy. Using a hot tub or sauna around the time your baby's neural tube should be closing may cause anencephaly. Having a high fever during the same time may also increase your baby's risk.

Diabetes. If you have uncontrolled diabetes, your blood sugar levels may get too high. This can cause problems with your baby's development.

Obesity. Being obese, or significantly overweight, may cause problems with your baby's development during the 4th week of gestation. 

Opioid use. One study found that women who took opioids in the first two months of their pregnancies were more likely to have babies with neural tube defects. These opioids include: 

Diagnosing Anencephaly

As part of your prenatal health care, your doctor may do tests to check for neural tube defects. Some of these tests include: 

Ultrasounds. An ultrasound uses sound waves to create pictures of your baby. Your doctor can look at these pictures to check your baby's brain, skull, and spine.  

Quad marker screen. A quad marker screen is a blood test to check for neural tube defects and genetic disorders. One of the tests checks for high levels of alpha-fetoprotein (AFP). Your baby's liver makes AFP. If your blood shows high levels of AFP in your blood, your baby may have anencephaly.   

Fetal magnetic resonance imaging (MRI). An MRI uses high-powered magnets to create detailed images of your baby's tissues and bones. Your doctor can use an MRI to get detailed images of your baby's spine and brain. This test provides more detail than an ultrasound.    

Amniocentesis. In amniocentesis, your doctor inserts a thin needle into the amniotic sac to get a sample of the fluid around your baby. A lab will check this fluid for high levels of AFP or acetylcholinesterase, which is a type of enzyme that shows that your baby may have a neural tube defect.  

Treating Anencephaly

Unfortunately, there is no treatment for anencephaly. Many pregnancies with neural tube defects will end in miscarriages. Some babies with anencephaly will be stillborn. Other babies who are born with anencephaly will die within a few hours to a few days. 

If your baby is born with anencephaly, your doctor can offer you resources to make the most of the time you have with your baby. 

Preventing Anencephaly

It's not always possible to prevent anencephaly. However, there are some ways you can reduce your baby's risk of having it: 

  • Take at least 400 micrograms of folic acid before and during your pregnancy.
  • Talk to your doctor about your medications if you are planning to become pregnant.
  • Avoid taking opioids if you think you may be pregnant. 
  • Avoid saunas and hot tubs if you think you may be pregnant. 
  • Talk to your doctor about reducing fevers while you're pregnant. 
  • Try to maintain a healthy weight before you get pregnant. 
  • If you have diabetes, talk to your doctor about the best way to manage it while you're pregnant. 

Show Sources


American Journal of Epidemiology: "Diabetes and Obesity-Related Genes and the Risk of Neural Tube Defects in the National Birth Defects Prevention Study."

Centers for Disease Control and Prevention: "Facts about Anencephaly," "Opioid Use and Neural Tube Defects."

Cleveland Clinic: "Anencephaly."

Fetal Health Foundation: "Anencephaly."

Lancet: "Maternal hyperthermia as a possible cause of anencephaly."

National Organization for Rare Disorders: "Anencephaly."

St. Louis Children's Hospital: "Anencephaly."

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