Anencephaly—a type of neural tube defect—is a fatal birth defect where the brain and skull do not form completely. About 36% of pregnancies with anencephaly are stillborn. In California, the defect is seen in about 1 in 3,880 births (including medically indicated pregnancy terminations).
Nearly 100% of cases can be detected during pregnancy with either the expanded AFP screening (a prenatal blood test) or ultrasound.The neural tube—the precursor to the brain and spinal cord—normally forms about 2-3 weeks after conception, often before a woman misses her period and realizes she might be pregnant. For this reason, we look for risk factors in the "periconception" period, including the month before the pregnancy began.