A 21-year-old woman, gravida 1 para 0, at 20 weeks gestation comes to the office for a fetal anatomy ultrasound. Her initial prenatal visit was last week, and she has had no vaginal bleeding, contractions, or leakage of fluid. The patient had not realized she was pregnant because her menses are irregular, typically occurring only once every 2 or 3 months. She has no known medical conditions and takes no medications. Blood pressure is 146/98 mm Hg and pulse is 88/min. BMI is 30 kg/m2. Ultrasound shows a large defect in the calvaria and meninges, with only a small cerebellum and brainstem, as seen in the image below.
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The remainder of the fetal anatomy appears normal. Amniotic fluid volume measures 26 cm (normal: <24 cm). Which of the following maternal factors is most commonly associated with this fetal anomaly?
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This fetus has anencephaly, a severe open neural tube defect (NTD) of the calvaria and skin leading to complete obliteration of the fetal cerebrum. Anencephaly is diagnosed by fetal ultrasound, which typically reveals an absent cerebrum, a calvarial defect, an abnormal cerebellum or brainstem, and elevated amniotic fluid volume (ie, polyhydramnios due to impaired fetal swallowing). Because of the severe amount of missing brain tissue, anencephaly is incompatible with life. Other NTDs (eg, myelomeningocele, in which the spinal cord and meninges are exposed) may be less severe and compatible with life.
NTDs occur when the fetal neural tube fails to close. Neural tube closure is a tightly regulated process dependent on folate metabolism. Therefore, inadequate maternal folate intake (ie, folate deficiency) is a major risk factor for NTDs. Because neural tube closure occurs at 5-6 weeks gestation, when most patients are unaware of their pregnancy (particularly if menses are irregular, as in this patient), all women should begin folic acid supplementation at least 1 month before conception to ensure adequate folate levels. Average-risk patients require 0.4 mg daily; high-risk patients (eg, prior affected pregnancy, antiepileptic use) require 4 mg daily.
(Choice B) Incomplete vaccination against rubella or varicella increases the risk of congenital infection, which can present with fetal growth restriction (rubella) or microcephaly (varicella) on ultrasound. Neither is associated with anencephaly.
(Choice C) Poorly controlled hypertension, as in this patient, increases the risk of fetal growth restriction, placental abruption, preeclampsia, and need for preterm delivery. It does not increase the risk of NTDs.
(Choice D) Undercooked meat consumption is associated with congenital toxoplasmosis, which can present with microcephaly, hydrocephalus, and diffuse cerebral calcifications on ultrasound. It does not cause NTDs.
(Choice E) Some untreated sexually transmitted infections such as syphilis and herpes simplex virus (HSV) increase the risk of congenital infection, which may present as hydrocephalus (syphilis) or microcephaly (HSV) on ultrasound. They are not associated with anencephaly.
Educational objective:
Inadequate maternal folate intake is a major risk factor for fetal neural tube defects (eg, anencephaly). Anencephaly is diagnosed by fetal ultrasound, which typically reveals an absent cerebrum, a calvarial defect, an abnormal cerebellum or brainstem, and polyhydramnios.