Anencephaly is a condition characterized by the absence of a major portion of the skull, brain, and scalp. The face and base of the skull appear almost normal in formation. Anencephaly is the most severe and common neural tube defect diagnosed prenatally, accounting for half of all cases of neural tube defects.
Anencephaly is considered the later gestation progression of acrania, as the exposed brain tissue that appears normal in acrania is broken down by amniotic fluid as the pregnancy progresses, resulting in no recognizable brain tissue. This progression from acrania to anencephaly is called the fetal acrania-anencephaly sequence.
Anencephaly can be detected as early as the first trimester via ultrasound and maternal bloodwork. An estimated 1 in 1,000 pregnancies is affected by anencephaly, but due to miscarriages and intrauterine deaths, the incidence rate of anencephaly is estimated to be 1 in 10,000 live births. Anencephaly has a stillbirth rate of around 20%; however, an estimated 72% of babies are born alive. Females are between three and four times more likely to be affected than males. This condition is considered uniformly fatal in the perinatal period.
Babies diagnosed with anencephaly have a mean gestation for delivery of 36 weeks, and their birth weights typically fall within the normal range for all babies born at that gestation. This gestation and birth weight means that babies born with anencephaly can be potential organ and tissue donors.
One-quarter of all pregnancies affected by anencephaly have the added complication of polyhydramnios. 13-33% of anencephaly cases have additional anomalies such as congenital diaphragmatic hernia, congenital heart defects, omphalocele, and other cranial defects. Anencephaly is not typically associated with aneuploidy, or chromosomal abnormalities.
The recurrence risk for anencephaly ranges from 2% to 5%. Parental genetic testing is recommended prior to subsequent pregnancies.
Getting adequate amounts of folate and folic acid can prevent neural tube defects like anencephaly. All women of childbearing age should get 400 micrograms (mcg) of folic acid daily, prior to and throughout pregnancy. For women who have experienced a prior pregnancy affected by neural tube defects, a high daily dose of 4 milligrams (mg) of folic acid is recommended, staring at least four weeks prior to conception.