Anencephaly can be detected as early as 10 to 14 weeks of pregnancy through ultrasound, making it one of the earliest structural abnormalities identified during prenatal care. Most cases are caught during routine first-trimester or second-trimester scans, and the condition is almost always confirmed before birth.
First-Trimester Ultrasound Detection
The earliest reliable window for detecting anencephaly is the 10 to 14-week ultrasound scan. A large study of over 55,000 pregnancies across seven hospitals found that anencephaly could be reliably diagnosed at this stage, as long as the sonographer specifically looked for the right signs. In the early phase of that study, about 74% of cases were caught during the first-trimester scan. After sonographers received additional training on what to look for in early pregnancy, the detection rate jumped to 100% in the second phase.
The reason detection requires some expertise at this stage is that anencephaly looks different in the first trimester than it does later. In early pregnancy, the brain tissue is still present but exposed to the surrounding amniotic fluid because the skull never fully formed. On a coronal ultrasound view, this creates a distinctive appearance sometimes called the “Mickey Mouse face” sign, where the exposed brain hemispheres sit visibly above the eye sockets. In one study, five out of six first-trimester cases showed this sign, with no false-positive diagnoses.
Second-Trimester Confirmation
The anatomy scan performed between 18 and 22 weeks remains the most common point of detection for many structural abnormalities, including anencephaly. By this stage, the exposed brain tissue has typically broken down further from contact with amniotic fluid, and the absence of the skull vault is unmistakable on ultrasound. Cases that weren’t caught at the earlier scan are almost always identified here. The classic appearance at this stage is quite different from the first-trimester signs, with prominent eye orbits and no visible brain or skull above them.
Blood Screening Between 16 and 20 Weeks
A blood test measuring a protein called alpha-fetoprotein (AFP) in the mother’s blood can also flag anencephaly. This protein leaks into the amniotic fluid and then into the mother’s bloodstream at abnormally high levels when the fetal skull and brain haven’t formed properly. The test is typically offered between 16 and 20 weeks of pregnancy.
In a study of nearly 11,600 pregnant women, AFP screening detected 93% of open neural tube defects. About 1.7% of pregnancies initially showed elevated levels, but most of those turned out to be false positives. After follow-up ultrasound or a repeat blood draw, the false-positive rate dropped significantly, and no normal pregnancies were terminated due to incorrect results. While this blood test is effective, ultrasound has largely become the primary tool for diagnosis because it can directly visualize the condition rather than relying on an indirect marker.
What Anencephaly Means for the Pregnancy
Anencephaly is a neural tube defect where the upper portion of the skull and brain fail to develop. It occurs very early in embryonic life, within the first few weeks after conception, when the neural tube (which eventually forms the brain and spinal cord) doesn’t close completely. By the time it’s visible on ultrasound, the condition is already fully established.
There is no treatment or surgical correction. According to the CDC, almost all babies born with anencephaly die shortly after birth, and pregnancy loss rates are high. Because of this prognosis, early detection gives families time to make informed decisions about the pregnancy and to access appropriate support and counseling.
Related Conditions That Look Similar
Very early in pregnancy, particularly around 8 to 11 weeks, anencephaly can be difficult to distinguish from related conditions along what’s called the acrania-exencephaly-anencephaly sequence. Acrania means the skull bones are absent but disorganized brain tissue is still visible above the eye sockets. As the unprotected brain degrades over time from amniotic fluid exposure, the condition progresses to exencephaly and eventually to anencephaly. Three-dimensional ultrasound can help differentiate between these stages, which matters for genetic counseling since related conditions like encephalocele (where brain tissue pushes through a gap in the skull) have a different outlook and different recurrence risks.
Reducing the Risk With Folic Acid
Taking 400 micrograms (0.4 mg) of folic acid daily before and during early pregnancy significantly reduces the risk of neural tube defects, including anencephaly. The CDC recommends this dose for all women of childbearing age who could become pregnant, since the neural tube closes so early that many women don’t yet know they’re pregnant when the critical window passes. Women who have had a previous pregnancy affected by a neural tube defect are advised to take a much higher dose of 4 mg daily, starting at least one month before conception and continuing through the first three months of pregnancy. Total daily folic acid intake should stay under 1 mg unless a doctor recommends otherwise.