When Is Anencephaly Detected During Pregnancy?

Anencephaly is typically detected during the first trimester, most often between 11 and 14 weeks of pregnancy. The clearest diagnosis usually comes from a blood test and ultrasound done in that window. In some cases, the condition is not seen until the 18 to 20 week anatomy scan.

Anencephaly is a serious neural tube defect. The neural tube is the structure that becomes the brain and spinal cord. When the upper part fails to close properly in early pregnancy, the brain and skull do not develop normally. This happens very early — by about the fourth week after conception, often before a person knows they are pregnant.

What Is Anencephaly and When Does It Develop?

Anencephaly is a birth defect in which major portions of the brain, skull, and scalp do not form. The neural tube normally closes within the first month after conception. In anencephaly, the upper end of that tube stays open.

Because this process happens so early, the defect is already present by the time most pregnancies are confirmed. That timing matters for detection. It means screening tests done in the first trimester can sometimes identify the problem.

Anencephaly is one of two main neural tube defects. The other is spina bifida, which affects the lower end of the neural tube. Anencephaly is the more severe of the two. Babies born with anencephaly are missing large parts of the brain and skull. Most do not survive long after birth.

Neural tube defects as a group are uncommon. Anencephaly specifically affects a small number of pregnancies each year in the United States. The exact rate has declined over recent decades, largely because of folic acid fortification of food and increased supplement use.

When Is Anencephaly Detected During Pregnancy?

Detection usually happens in two stages. The first is a first-trimester screening, done between 11 and 14 weeks. The second is the mid-pregnancy anatomy scan, done around 18 to 20 weeks.

The first-trimester screen combines a blood test with an ultrasound. The blood test measures certain proteins and hormones. The ultrasound measures the back of the baby’s neck, a space called the nuchal translucency. In anencephaly, the skull is not visible on ultrasound, and the nuchal measurement is often abnormal. These findings can prompt further evaluation.

Anencephaly is sometimes seen even earlier. By around 10 to 11 weeks, a skilled ultrasound operator may notice that the skull is missing. Before that point, the brain and skull are still developing, so the defect can be harder to identify with certainty.

The 18 to 20 week anatomy scan is the other common point of detection. This detailed ultrasound looks at the baby’s organs and structures. A missing skull and absent or minimal brain tissue are usually clear on this scan.

In some cases, anencephaly is found on a routine ultrasound done for another reason. Occasionally it is suspected because of an abnormal blood test result, and confirmed with imaging.

How Is Anencephaly Diagnosed?

Diagnosis relies on imaging, primarily ultrasound. On a standard ultrasound, the technician looks for the shape of the skull. In anencephaly, the top of the skull is absent. The brain tissue above the brainstem is largely missing.

Two features are usually described. One is the “frog eye” appearance, where the eyes appear to bulge because the skull above them is missing. The other is the absence of the cranial vault — the bony covering of the brain.

A detailed ultrasound can confirm the diagnosis in most cases. In some situations, a doctor may recommend a follow-up scan or a referral to a specialist in maternal-fetal medicine. This is done to confirm the finding and rule out other conditions that can look similar.

Blood tests cannot diagnose anencephaly on their own. They can raise suspicion by showing abnormal levels of certain markers. But the diagnosis itself rests on ultrasound findings.

In rare cases, anencephaly may be suspected based on an elevated alpha-fetoprotein level in the mother’s blood. Alpha-fetoprotein is a protein made by the fetus. In open neural tube defects, more of it crosses into the mother’s bloodstream. An elevated result is a signal to look more closely, not a diagnosis by itself.

What Causes Anencephaly?

Anencephaly happens when the neural tube fails to close. The exact reason for this failure is not fully understood in most cases. It appears to result from a mix of genetic and environmental factors.

Folic acid is the most well-established factor. Research consistently shows that low folic acid intake before and during early pregnancy raises the risk of neural tube defects, including anencephaly. This is why folic acid is added to many grain products in the United States and why supplements are recommended before pregnancy.

Other factors have been linked to higher risk in some studies. These include certain medications, diabetes that is not well controlled, obesity, and high body temperature early in pregnancy from fever or heat exposure. The evidence for each of these varies. None of them is a single cause on its own.

Some cases are linked to genetics. A small number of anencephaly cases are associated with specific gene changes or syndromes. For most families, though, there is no clear inherited pattern.

It is important to say plainly: in many cases, no specific cause is ever identified. Parents often want to know what they did wrong. In most situations, the answer is nothing identifiable.

What Does Anencephaly Mean for the Pregnancy?

Anencephaly is considered a fatal condition. Babies with anencephaly are missing the parts of the brain needed for consciousness and survival. Most are stillborn or die shortly after birth. A small number may live for a few days or, rarely, longer, but the condition is not compatible with long-term survival.

Because of this, the diagnosis carries serious decisions. Parents may be offered options including continuing the pregnancy with palliative care or choosing to end the pregnancy, depending on the stage and local laws. These are personal decisions, and care teams typically provide counseling and support.

Pregnancy with an anencephaly diagnosis can also carry risks for the mother. One is polyhydramnios, which is too much amniotic fluid. This happens because the baby has trouble swallowing, which normally helps regulate fluid levels. Polyhydramnios can cause discomfort and may need monitoring.

Some pregnancies with anencephaly end in miscarriage or stillbirth before the due date. Others continue to term. The course varies, and no single outcome applies to every case.

Can Anencephaly Be Prevented?

Not all cases can be prevented. But the risk can be reduced. The strongest step is getting enough folic acid before and during early pregnancy.

Folic acid is a B vitamin. The neural tube closes in the first month after conception, often before a person knows they are pregnant. That is why the recommendation is to take folic acid before pregnancy, not after a positive test.

For most people who could become pregnant, the standard recommendation is 400 micrograms of folic acid daily. Some people need a higher dose, such as those with a previous pregnancy affected by a neural tube defect, those with certain health conditions, or those taking certain medications. A higher dose should only be used under a doctor’s guidance.

Folic acid is found in fortified cereals, leafy green vegetables, beans, and citrus fruits. Many people do not get enough from food alone, which is why supplements are often recommended.

Other steps that may help include managing diabetes well before and during pregnancy, avoiding high body temperatures early in pregnancy, and talking to a doctor about any medications before becoming pregnant. These steps lower risk. They do not guarantee prevention.

What Should You Do If Anencephaly Is Suspected?

If a screening test or ultrasound raises concern, the next step is usually a detailed ultrasound. This may be done by a specialist in maternal-fetal medicine. The goal is to confirm the finding and get a clear picture.

Ask for a clear explanation of what the imaging shows. Ask what the diagnosis means for the pregnancy and what options are available. It is reasonable to request a second opinion or a referral to a center experienced with neural tube defects.

Counseling and support are part of care. Many hospitals have teams that help families through this diagnosis. Genetic counseling can help families understand recurrence risk if they plan future pregnancies.

If a pregnancy is affected, a doctor may recommend a higher dose of folic acid before any future pregnancy. This is a decision to make with a care team, not on your own.