A 36-year-old woman, gravida 1 para 0, comes to the office for an initial prenatal visit. The patient recently immigrated to the United States and has not received prenatal care. She is at 22 weeks gestation by a sure, regular last menstrual period. Vital signs are normal. Height is 152.5 cm (5 ft 0 in). BMI is 26 kg/m². Fundal height is 27 cm. Routine prenatal laboratory results include: Ultrasound reveals a male fetus with an edematous scalp and nuchal fold. The fetal abdomen contains a large amount of echolucent fluid. The biparietal diameter and femur length measure at 22 weeks gestation. The single deepest pocket of amniotic fluid is 12 cm, and the placenta is thickened to 6 cm. Fetal heart rate is 170/min. Which of the following is the most likely cause of this fetal presentation?

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11ec25c8_da33_3b3b_bc81_438b81b178ad_MD0003_00 This fetus has ascites (eg, echolucent abdominal fluid), skin edema, polyhydramnios (single deepest pocket ≥8 cm), and placental thickening (reflecting intravillous edema) consistent with hydrops fetalis (fetal hydrops); additional features may include pleural and pericardial effusion. Hydrops fetalis can occur due to a variety of disorders (eg, Rh alloimmunization, parvovirus B19). In this fetus with hydrops fetalis and no associated dysmorphic features (eg, normal biparietal diameter and femur length), the most likely cause is alpha-thalassemia major. fetal hydrops Normal hemoglobin is a heterotetramer composed of 2 alpha and 2 beta chains (hemoglobin A) in adults, or 2 alpha and 2 gamma chains (hemoglobin F) in fetuses. Patients with alpha-thalassemia have reduced or absent alpha chain synthesis, and the disease severity is determined by the number of affected genes. Most patients have alpha-thalassemia minor with mild anemia (as in this mother). However, when both parents have the "cis" (αα/- -) genotype of alpha-thalassemia minor, their fetus can inherit a loss of all 4 alpha-globin genes (- -/- -), leading to no alpha-globin chain production. Normal hemoglobin Without alpha chains, fetuses form gamma homotetramers (4 gamma chains) known as hemoglobin Barts, which has an extreme affinity for oxygen (>10 times more than hemoglobin A) and does not release oxygen to the tissues. This results in severe fetal hypoxemia, development of high-output cardiac failure, and subsequent hydrops fetalis and intrauterine fetal demise. (Choice A) Achondroplasia can present with hydrops fetalis, but affected fetuses typically have macrocephaly and shortened long bones on prenatal ultrasound. This fetus's biparietal diameter and femur length are normal. (Choice C) Beckwith-Wiedemann syndrome is associated with polyhydramnios; however, affected fetuses typically have macrosomia and omphalocele (not seen in this fetus). (Choice D) Cystic fibrosis may be associated with hyperechogenic bowel or absent gallbladder on prenatal ultrasound but not hydrops fetalis. (Choice E) Turner syndrome (45, X) can present with nuchal thickening, cystic hygroma (fluid-filled lymphatic sac), and hydrops fetalis. This diagnosis is unlikely because Turner syndrome only affects females. cystic hygroma Educational objective: Alpha-thalassemia major is an inherited disorder of absent alpha-globin chain production necessary to form hemoglobin F. Fetuses with alpha-thalassemia major produce hemoglobin Barts (4 gamma chains), which does not release oxygen to the tissues. This results in severe fetal hypoxemia, high-output heart failure, hydrops fetalis (eg, skin edema, ascites), and subsequent fetal demise. References: Etiology of nonimmune hydrops fetalis: an update. (http://www.ncbi.nlm.nih.gov/pubmed/25712632) Atypical prenatal ultrasound presentation and neuropathological findings in a neonate with alpha thalassemia major: a case report. (http://www.ncbi.nlm.nih.gov/pubmed/30550718) Alpha-thalassemia. (http://www.ncbi.nlm.nih.gov/pubmed/20507641)

A 36-Year-Old Woman, Gravida 1 Para 0, Comes to the Office