Anencephaly is a serious birth defect in which a baby is born without major parts of the brain and skull. It happens when the upper end of the neural tube, the structure that becomes the brain and spinal cord, fails to close during the first few weeks of pregnancy. The condition is uniformly fatal, with almost all affected babies dying shortly after birth. Globally, anencephaly occurs in roughly 5 to 8 out of every 10,000 births.
How Anencephaly Develops
Around the third and fourth week of pregnancy, a flat strip of cells along the embryo’s back folds inward and seals itself into a tube. The top of this tube becomes the brain; the bottom becomes the spinal cord. In anencephaly, the top portion never closes. Without this closure, the skull bones (called the calvarium) don’t form, and the brain tissue that would normally grow inside the skull is left exposed to amniotic fluid. Over time, the unprotected tissue breaks down.
The brainstem, which controls basic reflexes like breathing and heartbeat, often remains partially intact. This is why some babies with anencephaly can be born alive and may briefly breathe or respond to touch. But the cerebral hemispheres, the parts of the brain responsible for thought, sensation, and consciousness, are absent or nearly absent.
What Anencephaly Looks Like
The condition is visible immediately at birth. The top of the head has no skull bone and no scalp covering it. In place of normal brain tissue, there is typically a mass of reddish, disorganized tissue sitting at the base of the skull. The eyes may appear prominent or bulging because the bony structures above the eye sockets are underdeveloped. On prenatal ultrasound, this creates a distinctive appearance sometimes called the “frog eye” sign, where the protruding eyes are visible without any skull structures above them.
How It Is Detected During Pregnancy
Anencephaly is one of the birth defects most reliably caught before birth. There are two main screening methods.
A blood test measuring alpha-fetoprotein (AFP), a protein produced by the developing baby, is typically done between 15 and 20 weeks of pregnancy. Higher than normal levels suggest an increased risk of a neural tube defect like anencephaly, though the blood test alone doesn’t confirm a diagnosis. It flags the need for further testing.
Ultrasound provides the definitive diagnosis. The absence of the skull is clearly visible on imaging, often detectable by the end of the first trimester. In a cross-sectional view of the head, irregular masses of tissue can be seen floating above the fetal eye sockets with no cranial structures surrounding them. Because the findings are so distinctive, the diagnosis is considered highly reliable once seen on ultrasound.
Known Risk Factors
The single most established preventable risk factor is low folate (vitamin B9) intake around the time of conception. Other factors that increase risk include:
- Maternal diabetes: Both pre-existing type 1 and type 2 diabetes are associated with higher rates of neural tube defects.
- Obesity: Higher pre-pregnancy body weight raises the risk, independent of folate levels.
- Overheating in early pregnancy: Exposure to hot tubs, saunas, or high fevers during the first trimester has been linked to neural tube defects. Research from a Georgia-based study found that mothers exposed to six or more consecutive days of extreme heat around conception had 29% higher odds of having a baby with a neural tube defect. A core body temperature above 39°C (about 102°F) is considered the threshold where fetal damage becomes likely.
- Previous affected pregnancy: Having one pregnancy affected by a neural tube defect significantly raises the chance of it happening again.
Genetics also play a role, though anencephaly doesn’t follow a simple inheritance pattern. It results from a combination of genetic susceptibility and environmental factors, which is why prevention efforts focus on the modifiable risks.
The Role of Folic Acid in Prevention
Taking 400 micrograms of folic acid daily before and during early pregnancy is the most effective way to reduce the risk of neural tube defects, including anencephaly. The U.S. Public Health Service recommends that anyone who could become pregnant take this amount, because the neural tube closes so early in pregnancy (often before a person even knows they’re pregnant) that waiting until a positive test is too late.
For women who have already had a pregnancy affected by a neural tube defect, the recommended dose jumps to 4,000 micrograms daily, starting at least one month before conception and continuing through the first three months of pregnancy. This tenfold increase reflects the substantially higher recurrence risk in these cases.
Prognosis and What Families Face
There is no treatment or cure for anencephaly. The rate of pregnancy loss is high, meaning many affected pregnancies end in miscarriage or stillbirth. Among babies born alive, survival is measured in hours to days, not weeks. Almost all die shortly after birth.
When families choose to continue the pregnancy, care after delivery focuses entirely on comfort. Some families take their baby home with hospice support. Research into the experiences of these families found that their primary concerns included feeding, managing the baby’s pain, changing dressings over the cranial defect, and navigating the uncertainty of how long their baby would live. Parents in these situations consistently described wanting to facilitate what they called “a good death” for their child.
One challenge families encounter is that many hospice providers who serve newborns with anencephaly lack specific training in perinatal bereavement. Parents may need to actively seek out bereavement resources, including online support communities and organizations specializing in perinatal loss, to find the kind of support that matches their experience.
Emotional Considerations for Families
Receiving an anencephaly diagnosis, whether during pregnancy or at birth, is devastating. Families face difficult decisions about continuing or ending the pregnancy, and there is no single right answer. Some parents choose to carry to term so they can meet their baby, hold them, and say goodbye. Others choose to end the pregnancy to avoid prolonged grief or medical risk. Both paths are common, and both come with their own emotional weight.
Grief after anencephaly can be complicated by the fact that many people in a family’s social circle don’t fully understand the condition or may not know how to respond. Connecting with other parents who have been through the same experience, through organizations focused on neural tube defects or perinatal loss, is one of the most consistently helpful steps families report taking.