Anencephaly: What It Is, Why It Happens, and What Families Should Know
Updated: 2 days ago
Medically reviewed by Dr. Baraa Alnahhal, MD · Last reviewed: September 2026
TL;DR
Anencephaly is a fatal birth defect in which a baby is born without parts of the brain and skull. It happens when the neural tube — the tissue that grows into the brain, skull, backbones, and spinal cord — fails to close at the top during the third and fourth weeks of pregnancy, before many women even know they are pregnant. Studies estimate it occurs in about 1 in 1,000 pregnancies, but because most affected pregnancies end in miscarriage or stillbirth, it affects an estimated 1 in 10,000 newborns in the United States. There is no cure or treatment; almost all babies born with anencephaly live only minutes, hours, or days, and they cannot feel pain. The single most important prevention step is taking 400 micrograms of folic acid daily before trying to conceive and during pregnancy.
Quick Answer
What it is: A fatal birth defect where a baby is born without parts of the brain and skull — a neural tube defect (NTD).
How common: About 1 in 1,000 pregnancies; roughly 1 in 10,000 U.S. newborns, because most affected pregnancies end in miscarriage or stillbirth.
How it happens: The top of the neural tube fails to close during weeks 3–4 of pregnancy, so the forebrain and cerebrum do not grow.
Prevention: 400 mcg of folic acid daily, started before trying to conceive — neural tube defects form in the first month of pregnancy, before pregnancy is often known.
What Is Anencephaly?
Anencephaly is a birth defect (congenital disorder) where a baby is born without parts of their brain and skull. The condition affects the nervous system, which includes the brain, spine, and nerves.
It belongs to a group of conditions called cephalic disorders, and more specifically it is a neural tube defect (NTD). The neural tube is the structure responsible for growing the brain, skull, backbones, and spinal cord.
The brain is essential to survival — it controls body functions, emotions, and memory. Because anencephaly disrupts how the brain develops, babies born with the condition usually live only a few minutes, hours, or days. Most pregnancies with anencephaly end in miscarriage or stillbirth.
Key Fact | Detail |
Category | Fatal birth defect; cephalic disorder; neural tube defect (NTD) |
What is missing | Parts of the brain and skull |
Body system affected | Nervous system (brain, spine, nerves) |
When the defect occurs | Third and fourth week of pregnancy |
Outcome | Almost all affected babies live minutes, hours, or days |
What Are the Types of Anencephaly?
There are three types of anencephaly, and all three are fatal for the fetus. The differences lie in how much of the brain, skull, and spine fails to form.
Meroanencephaly is the type in which the brainstem and midbrain only partially develop, with some skin and skull covering the brain. Holoanencephaly, the most common type, means the brain did not develop at all. Craniorachischisis is the most severe type — the brain, skull, and spine did not develop.
Type | What Develops | Distinction |
Meroanencephaly | Brainstem and midbrain partially develop; some skin and skull cover the brain | Partial development |
Holoanencephaly | Brain did not develop at all | Most common type |
Craniorachischisis | Brain, skull, and spine did not develop | Most severe type |
How Common Is Anencephaly?
Anencephaly is a relatively common type of neural tube defect. Studies estimate it happens in 1 in 1,000 pregnancies. However, because most pregnancies with anencephaly end in miscarriage, the condition affects an estimated 1 in 10,000 newborns in the United States.
What Are the Signs of Anencephaly?
The birth mother may not notice any symptoms on her own. Anencephaly is typically discovered through blood tests or prenatal imaging. Signs include:
Sign | How It Is Found |
High levels of alpha-fetoprotein (AFP) | A fetal protein detected in a maternal blood test or amniotic fluid sample; the blood test is usually done in the second trimester |
Polyhydramnios | Too much fluid in the amniotic sac, seen on a prenatal ultrasound |
Missing parts of the skull and brain | Seen on imaging |
Exposed brain tissue | Areas with no skin or skull covering, seen on imaging |
Smaller head size than expected | Seen on imaging |
What Can a Baby with Anencephaly Experience?
Newborns with anencephaly do not have consciousness (awareness), vision, hearing, or the ability to feel pain. This last point matters deeply for families.
Not all newborns with anencephaly develop a brainstem. If a brainstem forms, the baby may show reflexes and automatically respond to touch. Seeing a newborn respond this way might give parents hope. But this is not a sign that the baby is aware of the touch or that they can live long-term with the condition — the largest part of the brain, the cerebrum (which allows movement, thinking, feeling, and responding), has not developed.
What Causes Anencephaly?
A problem with how the neural tube forms and closes causes anencephaly. The neural tube starts as a flat piece of tissue that grows into a tube shape, with each part responsible for different body structures:
Neural Tube Region | What It Grows Into |
Top | Brain and skull |
Middle | Spinal cord |
Bottom | Backbones |
Anencephaly happens when the top part of the neural tube does not close during embryonic development, during the third and fourth week of pregnancy. The fetus continues to form, but the front of the brain (forebrain) and the top of the brain (cerebrum) do not grow. Other parts of the brain may develop as expected, but no skin or skull covers them.
Is Anencephaly Inherited?
As far as medical knowledge goes, anencephaly is not inherited (passed down in families). In most cases it occurs without any family history of the condition — a sporadic genetic mutation.
There is one important exception. If you previously had a child with a neural tube defect, such as spina bifida, you have a higher chance of having a baby with anencephaly. That recurrence risk is about 2% to 3% — roughly 20 times higher than the risk for someone who has not had a previous baby with spina bifida.
What Are the Risk Factors for Anencephaly?
Certain medications and risk factors increase the chance of having a baby with anencephaly or another neural tube defect:
Risk Factor | Why It Matters |
Lack of folic acid | Not getting enough folic acid (vitamin B9) during pregnancy raises the risk; providers recommend a prenatal vitamin with 400 mcg of folic acid before and during pregnancy |
Diabetes | Changes to blood glucose levels can be dangerous for a developing fetus |
Certain medications | Antiseizure medications such as phenytoin (Dilantin), carbamazepine (Tegretol), and valproic acid (Depakote) increase NTD risk; some also treat migraines and bipolar disorder |
Opioid use | Taking opioids during the first two months of pregnancy can cause NTDs, including heroin and prescription painkillers such as hydrocodone |
If you take any of these medications and plan to become pregnant, talk to your healthcare provider about your current medications — but do not stop taking medication unless your provider approves it.
How Is Anencephaly Diagnosed?
Prenatal screening tests can detect birth defects and other conditions that might affect the health of the fetus. The tests used to diagnose anencephaly include:
Test | How It Works | What It Detects |
Quad marker screen | Blood test sent to a lab; one of its four markers is alpha-fetoprotein (AFP) | High AFP levels, which leak into the mother's blood when anencephaly is present; AFP can also be tested alone |
Ultrasound | Sound waves produce pictures of the fetus | The fetal skull, brain, and spine |
Fetal MRI | High-powered magnets produce images of tissues and bones | The brain and spine in greater detail |
Amniocentesis | A thin needle withdraws fluid from the amniotic sac | High AFP and the enzyme acetylcholinesterase; either may indicate a neural tube defect |
If you opt out of prenatal screening, a provider may diagnose anencephaly after birth during the newborn's physical exam.
How Early Can It Be Detected?
Anencephaly may be detectable after eight to 12 weeks in the first trimester. On average, screenings for anencephaly happen around 18 to 20 weeks.
How Is Anencephaly Treated?
There is no available cure or treatment for anencephaly. Almost all babies born with anencephaly die within a few hours or days after birth. The care team's role is to support the family — helping parents grieve, say goodbye, and receive follow-up care.
Does a Baby with Anencephaly Feel Pain?
You may worry that a newborn with anencephaly is distressed, uncomfortable, or in pain. Babies with anencephaly cannot feel pain.
Infants who survive birth may seem to respond to touch or sound, but they are not truly responding to feeling or sound — they do not have the cerebrum, which allows them to move, think, feel, and respond.
Can Anencephaly Be Prevented?
It is not always possible to prevent anencephaly, but you can reduce your chance of having a child with the condition:
Prevention Step | Details |
Get plenty of folic acid | Take 400 mcg of folic acid daily, even if you do not plan to get pregnant right away. NTDs form in the first month of pregnancy — before you know you are pregnant. Start before trying to conceive. If you previously had a child with an NTD, a higher dose is usually recommended |
Review your medications | Some antiseizure and other medications can cause NTDs; discuss changes with your provider before becoming pregnant |
Manage your health | If you have a condition like diabetes, work with your provider to manage it before and during pregnancy |
How Do You Take Care of Yourself After This Loss?
Learning that your baby has anencephaly is extremely difficult. Your care team will help you and your loved ones say goodbye and provide follow-up care. You may need support or someone to talk to — your team can connect you to a mental health professional or a bereavement support group where you can share your feelings with others who have had similar losses.
It is understandable to feel heartbroken and hopeless, but surrounding yourself with a supportive network can help with the grief you are feeling.
When Should You See a Healthcare Provider?
If you are planning to become pregnant, start with a preconception counseling visit. A provider can help you maintain good health to lower your risk of having a child with a birth defect.
If you are pregnant and experience symptoms of a miscarriage — heavy bleeding, cramps, and/or severe back or abdominal pain — contact your pregnancy care provider right away.
Conclusion
Anencephaly is one of the most difficult diagnoses a family can face, and it deserves to be understood clearly. It is a fatal neural tube defect caused when the top of the neural tube fails to close during the third and fourth weeks of pregnancy — leaving the forebrain and cerebrum undeveloped and uncovered by skin or skull. It is estimated at 1 in 1,000 pregnancies, and nearly all affected pregnancies end in miscarriage or stillbirth, with the few babies born alive surviving only minutes to days.
The medical facts are sobering, but two truths deserve emphasis. First, babies with anencephaly cannot feel pain, which offers families one measure of comfort. Second, the risk of neural tube defects can be meaningfully reduced: 400 mcg of folic acid daily, started before conception, a careful review of medications, and well-managed health conditions. For families who have lost a child to this condition, grief support and preconception counseling for a future pregnancy are the most important next steps.
If you are planning a pregnancy, book a preconception counseling visit — ask specifically about the right folic acid dose for you, especially if you have had a previous pregnancy affected by a neural tube defect, take antiseizure medication, or manage diabetes. Starting 400 mcg of folic acid before conception is the single most effective step you can take to protect your baby's neural tube.
FAQ
What is anencephaly?
Anencephaly is a fatal birth defect where a baby is born without parts of the brain and skull. It is a neural tube defect that affects the nervous system, and it happens when the top of the neural tube fails to close during the third and fourth week of pregnancy.
How common is anencephaly?
Studies estimate anencephaly happens in about 1 in 1,000 pregnancies. Because most affected pregnancies end in miscarriage or stillbirth, it affects an estimated 1 in 10,000 newborns in the United States.
What are the types of anencephaly?
There are three types, all fatal for the fetus. Meroanencephaly: the brainstem and midbrain partially develop, with some skin and skull covering. Holoanencephaly: the brain did not develop at all (most common). Craniorachischisis: the brain, skull, and spine did not develop (most severe).
What are the signs of anencephaly during pregnancy?
The mother usually has no symptoms herself. Signs appear on tests: high alpha-fetoprotein (AFP) in maternal blood or amniotic fluid, polyhydramnios (too much amniotic fluid), missing parts of the skull and brain, exposed brain tissue, and a smaller-than-expected head.
How early can anencephaly be detected?
It may be detectable after 8 to 12 weeks of the first trimester, though on average screenings happen around 18 to 20 weeks of pregnancy.
Is anencephaly inherited?
No. As far as medical knowledge goes, anencephaly is not passed down in families and usually occurs without any family history, as a sporadic genetic mutation. However, having a previous child with a neural tube defect raises the recurrence risk to about 2%–3% — 20 times higher than usual.
Can a baby with anencephaly feel pain?
No. Babies with anencephaly cannot feel pain. They also lack consciousness, vision, and hearing. If a brainstem forms, the baby may show reflex responses to touch, but these are not signs of awareness or of long-term survival.
Can anencephaly be prevented?
Not always — but risk can be reduced by taking 400 mcg of folic acid daily before and during pregnancy (starting before trying to conceive, since NTDs form in the first month), reviewing medications with a provider, and managing health conditions such as diabetes. A higher folic acid dose is usually recommended after a previous child with a neural tube defect.
References
Centers for Disease Control and Prevention (U.S.). "Facts About Anencephaly." Last reviewed 12/16/2022. cdc.gov/ncbddd/birthdefects/anencephaly.html
Khan IA, Firdaus U, Ali SM, et al. "Newborn with Meroanencephaly: Surviving All Odds." J Pediatr Neurosci. 2016 Jul-Sep;11(3):228-229. ncbi.nlm.nih.gov/pmc/articles/PMC5108127
March of Dimes Foundation. "Neural Tube Defects." Last reviewed 02/01/2022. marchofdimes.org/find-support/topics/planning-baby/neural-tube-defects
National Institute of Neurological Disorders and Stroke (U.S.). "Anencephaly." ninds.nih.gov/health-information/disorders/anencephaly
National Library of Medicine (U.S.). "Anencephaly." 10/01/2019. medlineplus.gov/genetics/condition/anencephaly
National Library of Medicine (U.S.). "Neural Tube Defects." Last reviewed 09/29/2016. medlineplus.gov/neuraltubedefects.html
National Organization for Rare Disorders, Inc. (U.S.). "Anencephaly." Last reviewed 08/01/2012. rarediseases.org/rare-diseases/anencephaly
Disclaimer: This article is for general educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for guidance specific to your situation.
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