Anencephaly

What happens in anencephaly

About 3 to 4 weeks after conception, a flat strip of cells in the embryo rolls up and closes to form the neural tube. The front end becomes the brain and the rest becomes the spinal cord. In anencephaly, the front end does not close. The developing brain is exposed to amniotic fluid and breaks down, and the bones of the top of the skull do not form.

A baby with anencephaly usually has a functioning brainstem but no forebrain or cerebrum, the parts responsible for thinking, awareness and coordination. Some babies have reflexes such as breathing and responding to touch, but they cannot become conscious.

Spina bifida and encephalocele are related neural tube defects affecting other parts of the tube.

Causes and risk factors

The exact cause in each pregnancy is usually unknown. Anencephaly results from a mix of genetic and environmental factors. Known risk factors include:

• Low folate: not getting enough folic acid before and in early pregnancy is the most important modifiable factor. See folate deficiency.
• A previous pregnancy with a neural tube defect, which raises the risk in future pregnancies.
• Certain medicines: valproic acid and carbamazepine for epilepsy, and some other drugs that interfere with folate.
• Poorly controlled diabetes before and in early pregnancy.
• Obesity before pregnancy.
• High body temperature in early pregnancy, such as from prolonged fever or hot tubs.
• Ethnicity and geography: rates differ between populations, partly due to diet and genetics.

How anencephaly is diagnosed

Anencephaly is almost always diagnosed before birth.

• Ultrasound: the absence of the skull vault is visible on scans from around 11 to 14 weeks and is reliably detected at the routine anatomy scan at 18 to 22 weeks.
• Maternal blood screening: alpha-fetoprotein (AFP) leaks from the exposed neural tissue into the amniotic fluid and the mother's blood. A high level on second-trimester screening (around 15 to 20 weeks) prompts a detailed scan.
• Amniocentesis is occasionally used, for example to look for chromosome conditions.

Excess amniotic fluid (polyhydramnios) often develops in later pregnancy because the baby cannot swallow normally.

Treatment and care

There is no treatment for anencephaly. After diagnosis, parents meet with obstetric and genetic specialists to discuss the diagnosis and their options. Depending on local laws and personal values, these include ending the pregnancy or continuing it with perinatal palliative care.

For families who continue the pregnancy:

• Prenatal care continues, with monitoring for polyhydramnios and preterm labor.
• Birth planning: a birth plan covering where to deliver, who will be present, and the family's wishes for time with the baby, photographs, cultural or religious rituals and memory-making.
• Comfort care after birth: warmth, feeding if possible, and relief of discomfort, without resuscitation or life-prolonging treatment.
• Organ and tissue donation may be possible in some cases. Ask the care team.
• Bereavement support for parents and siblings.

Outlook

Anencephaly is fatal. Many affected pregnancies end in miscarriage or stillbirth. Babies born alive usually live a few hours to a few days, rarely longer. They are unable to gain consciousness.

Preventing anencephaly in future pregnancies

• Folic acid for everyone who could become pregnant: the CDC recommends 400 micrograms (mcg) of folic acid every day, from at least 1 month before conception through early pregnancy, in addition to food sources. Because the neural tube closes so early, starting only once pregnancy is confirmed may be too late.
• Higher dose for high-risk pregnancies: people who have had a pregnancy affected by a neural tube defect, or who have certain other risk factors, are advised to take a much higher prescribed dose. Talk to your doctor before trying to conceive.
• Review medicines with your doctor before pregnancy, especially anti-seizure medicines.
• Control diabetes and aim for a healthy weight before pregnancy.
• Treat fever promptly in early pregnancy and avoid hot tubs and saunas.
• Food fortification: in the US, enriched grain products have been fortified with folic acid since 1998, which has reduced neural tube defects.

Genetic counseling helps families understand their chance of recurrence and plan future pregnancies.

Frequently asked questions

What is anencephaly?

Anencephaly is a birth defect in which a baby is born without large parts of the brain and the top of the skull. It happens when the upper end of the neural tube fails to close about 3 to 4 weeks after conception. It is one of the most severe neural tube defects.

Can a baby with anencephaly survive?

No. Anencephaly is always fatal. Many pregnancies end in miscarriage or stillbirth. Babies who are born alive usually live for hours to a few days. Care after birth focuses on comfort and giving the family time with their baby.

What causes anencephaly?

There is no single cause. Low folic acid levels before and in early pregnancy, a previous affected pregnancy, some anti-seizure medicines, poorly controlled diabetes, obesity and high fever in early pregnancy all raise the risk. In most cases, the parents did nothing to cause it.

Can anencephaly be prevented?

The risk can be greatly reduced but not completely eliminated. Taking 400 mcg of folic acid daily, starting at least a month before conception, lowers the risk of neural tube defects. People with a previous affected pregnancy need a higher prescribed dose, so they should see a doctor before trying to conceive.

When is anencephaly detected?

It is usually detected on ultrasound in the first trimester, around 11 to 14 weeks, and almost always by the routine anatomy scan at 18 to 22 weeks. A raised alpha-fetoprotein level on mid-pregnancy blood screening can also prompt a detailed scan.