A 37-year-old gravida 2, para 1 woman had a fetal anatomic survey at 20 weeks and 3 days’ gestation that identified a uterine mass. The pregnancy had been uncomplicated, with unremarkable findings on prenatal laboratory testing with low-risk cell-free DNA and normal nuchal translucency scan. The patient’s first pregnancy had resulted in a term cesarean delivery and had been complicated by gestational hypertension. Her medical history was noncontributory. Her medications included prenatal vitamins and aspirin for preeclampsia prophylaxis.The anatomic survey revealed a viable fetus with normal anatomy, cardiac activity, fetal movement, and amniotic fluid volume. Placentation was normal, with central insertion of a 3-vessel umbilical cord. In addition, a mass measuring 5.4 × 2.7 × 2.4 cm was detected in the right upper quadrant of the uterus adjacent to the placenta (Fig 1). Skeletal contents, including ribs and a spine, were visualized within the mass. A dividing membrane was seen. No blood flow was identified on color Doppler to the mass or within the mass. Given the identification of an extraneous intrauterine mass, the patient was referred to the center’s maternal-fetal medicine (MFM) service for consultation.The differential diagnosis of a pregnancy with a newly identified intrauterine mass containing skeletal contents includes twin death and less likely, an acardiac acephalic amorphous twin or a placental teratoma (Table 1). At this patient’s initial MFM evaluation, she was counseled about these potential diagnoses. The MFM physician explained to the family that the most common diagnosis is a twin death, wherein an intrauterine death of 1 of 2 appropriately developed fetuses in a twin gestation is seen. This can occur in 2% to 5% of twin pregnancies and is seen in both monochorionic and dichorionic pregnancies.An acardiac twin is observed exclusively in monochorionic/monozygotic pregnancies. In this scenario, 1 twin forms a rudimentary heart (or none at all) and lacks other structures essential for ex utero survival. In addition, a poorly developed umbilical cord, often composed of 1 artery and 1 vein, connects the acardiac twin to the shared placenta. (1) In these monochorionic pregnancies, the acardiac twin is perfused by its viable cotwin (commonly referred to as the “pump twin”) via aberrant arterioarterial placental anastomoses. This is known as twin reversed arterial perfusion (TRAP) sequence, and the pathognomonic ultrasound finding is retrograde flow in the umbilical artery of the acardiac twin (ie, with blood flow coming from the placenta to the acardiac twin via the umbilical arteries). (2)(3) TRAP sequence is an extreme form of twin-to-twin transfusion syndrome (TTTS), which is also a potential complication of monochorionic twinning with unbalanced arteriovenous placental anastomoses. In classic TTTS, 1 twin receives an excessive share of the pooled blood and becomes polycythemic, while the cotwin is anemic. In contrast to TRAP sequence, both twins are fully developed anatomically in TTTS.Teratomas are typically benign, nontrophoblastic neoplasms that arise from germ cells. (4) Their composition varies from immature to well-differentiated tissues that are foreign to the anatomic site in which the mass is found. The placenta is an extremely rare site for this tumor, with only a few cases reported in the literature. (4)(5) In contrast to an acardiac twin, placental teratomas are not attached to the placenta via an umbilical cord and do not display any degree of tissue organization. (4)(5)The patient underwent serial imaging. Ultrasonography at 28 weeks’ gestation demonstrated interval growth of the mass to 8.3 × 6.6 cm (Fig 2). This imaging excluded the death of a cotwin as a diagnosis, as growth would not be observed on follow-up ultrasonography. Skeletal contents, including spine, ribs, and long bone, were again visualized. No upper body structures were observed. A vessel with blood flow was identified via color Doppler for the first time, possibly representing a rudimentary umbilical cord. This finding suggested that the mass was more likely an acardiac twin. The viable pump twin demonstrated normal interval growth, cardiac activity, amniotic fluid volume, and fetal movement.The patient’s MFM physician consulted with specialists at a referral center that surgically treats complications of monochorionic twin gestations, including acardiac twins. Because of the pump twin having to perfuse itself, as well as the acardic cotwin, acardiac twin pregnancies can be complicated by anemia and high-output heart failure of the pump twin. As such, the consultants recommended twice-weekly ultrasound surveillance to monitor growth and to look for signs of anemia (eg, elevated middle cerebral artery Doppler flow, polyhydramnios, hydrops) or cardiac decompensation (eg, reversal of flow in the ductus venosus) in the viable fetus. They recommended delivery at 34 to 36 weeks’ gestation at a regional perinatal health center with the possibility of delivery at an earlier gestation if the viable twin became compromised.At 29 weeks’ gestation, fetal ultrasonography demonstrated continued growth of the acardiac twin, along with normal Doppler studies and no evidence of compromise in the pump twin. At 30 weeks’ gestation, ultrasonography showed a 2-vessel umbilical cord attached to the acardiac twin with retrograde flow in the umbilical artery, consistent with a diagnosis of TRAP sequence. The viable pump twin continued to have normal amniotic fluid volumes, biophysical profiles, and Doppler studies in the middle cerebral artery, umbilical artery, umbilical vein, and ductus venosus. There was no evidence of fetal anemia or hydrops in the viable pump twin.At 31 weeks’ gestation, fetal echocardiography of the pump twin demonstrated a structurally normal heart with no signs of significant valve dysfunction. A patent foramen ovale and ductus arteriosus with right-to-left flow were visualized, along with qualitatively good biventricular systolic function.At 31 weeks and 2 days’ gestation, fetal ultrasonography demonstrated findings concerning for worsening TRAP sequence (Fig 3). The acardiac twin had grown to 11.6 × 10.5 × 8.2 cm with increasing edema and continued to demonstrate retrograde flow in the umbilical artery of a 2-vessel cord. In addition, an intermittent reversed A-wave was noted in the ductus venosus of the pump twin, indicating a potential early sign of cardiac compromise. The pump twin continued to have normal amniotic fluid volume, biophysical profile, and Doppler studies with no signs of fetal anemia or hydrops. A course of betamethasone was initiated at 31 weeks and 2 days’ gestation, and daily ultrasonography was performed due to the threat of preterm delivery and progression of TRAP sequence.The MFM physician continued to counsel the family about the timing of delivery, with the goal of minimizing compromise to the viable twin from TRAP sequence, while balancing the risk of complications from preterm delivery. The patient also received a neonatology consultation to gain greater insight into the potential consequences of delivery at 32 versus 34 weeks’ gestation. The neonatologist discussed the clinical outcomes and typical needs of infants in this moderate preterm period, including respiratory and feeding support. The neonatologist recommended postnatal echocardiography to assess for cardiac dysfunction given the increasing size of the acardiac twin and thus the potential for an increased hemodynamic burden in the pump twin. Because of concerns for the ongoing in utero health of the viable twin, given the possible finding of a reversed A-wave in the ductus venosus of the pump twin suggesting possible evolving cardiac failure, the obstetrical team, with input from the neonatologists and family, decided to deliver the patient at 32 weeks’ gestation.Before delivery, a multidisciplinary team huddle was held to discuss expectations regarding the appearance of the acardiac twin. Delivery was performed at 32 weeks and 1 day of gestation via cesarean delivery due to breech presentation of the viable twin. A female infant emerged with good tone and cry, and cord clamping was delayed by 30 seconds. The infant initially only required routine drying and stimulation. However, continuous positive airway pressure (CPAP) was started at approximately 3 minutes of age for increased work of breathing and persistent cyanosis. The infant’s Apgar scores were 8 and 8 at 1 and 5 minutes, respectively. Her birthweight was 2,160 g (89th percentile for gestational age), head circumference was 31.5 cm (96th percentile for gestational age), and length was 45 cm (91st percentile for gestational age).The infant was admitted to the NICU due to prematurity and respiratory distress. Physical examination findings were notable for a pink, well-perfused infant with a holosystolic murmur, mild subcostal retractions with CPAP in place, and no edema, ascites, or organomegaly. Chest radiography on admission showed diffuse bilateral interstitial and granular opacities compatible with surfactant deficiency and microatelectasis without cardiomegaly. On the first day after birth, echocardiography performed to assess for signs of high-output cardiac failure showed a moderate atrial septal defect with good biventricular function. The infant continued to receive CPAP until the second day after birth when she was transitioned to room air. She did not require caffeine. Her complete blood cell count soon after birth showed mild anemia with a hematocrit of 36.5%. Electrolytes on the second day after birth showed mild acute renal injury with a creatinine of 1.0 mg/dL (88.4 μmol/L), which normalized to 0.6 mg/dL (53.1 μmol/L) before discharge. Cranial ultrasonography performed on the 8th day after birth did not show any abnormalities. The infant spent a total of 5 weeks in the NICU due to immature feeding and respiratory control before being discharged with her parents.The placenta and attached acardiac twin was sent to pathology for further analysis. Gross pathologic examination revealed a 392-g monochorionic placenta with 1 true umbilical cord, as well as a vascular connection to the acardiac twin (Fig 4). The placenta was less than the 10th percentile in size for a 32-week twin gestation. The pump twin’s membranes were 100% marginal. The 3-vessel umbilical cord of the pump twin was inserted centrally, and vessels could be traced along the fetal surface of the placenta. Arterial-arterial anastomoses and venous-venous anastomoses between the pump twin’s umbilical cord and the vasculature connection of the acardiac twin to the placenta were identified via perfusion of the umbilical artery with milk (Fig 5). Of note, the milk was selected so that the flow within the blood vessels can be easily tracked. Milk is typically preferred over black ink as it does not stain vessels nor does it obscure anatomy if it leaks out of the vessel.Gross examination of the acardiac twin revealed a folded, fleshy mass, measuring 15.7 × 11.6 × 9.3 cm, encased within its own amniotic sac. Stripping of the membranes revealed less than 1 mL of clear fluid. The acardiac twin was attached to the placenta via a pair of unprotected vessels, measuring 0.2 cm in diameter overall. These vessels passed through the wall of the amniotic sac and were closely tethered to it. After traversing the membrane, the tissue surrounding the vessels became edematous, mimicking the appearance of Wharton jelly, and measured up to 0.5 cm in diameter overall. Insertion of the vessels was central and superior to the large fold that separated the body from a single limb. The rostral aspect of the acardiac twin displayed light-colored lanugo, but there was no development of features of a head or face; the caudal aspect had a single leg with a portion of a foot with 2 small toes (Fig 6). A small nub suggestive of a primitive upper limb was also present on the ventral surface, superior and lateral to the insertion of the vessels. The specimen could not be straightened or unfolded and showed little flexibility overall.Radiographs were taken before dissection (Fig 7). A calcified spine with dysmorphic vertebrae and short ribs was attached to a calcified and dysmorphic pelvis. A femur, tibia, and fibula were identified within the single leg. Leg bones were widely separated from each other. A ball and socket hip joint was identified at the location of the fold in the specimen. In addition, a small bone fragment was seen within the primitive upper limb.Dissection of the acardiac twin (Fig 8) showed an edematous mass mainly composed of skin and subcutaneous tissue. The spine, ribs, ball and socket joint of the hip, and leg bones were identified. The insertion of the umbilical cord was dissected to reveal a tangled mass of dilated vessels. No organs were grossly identified at dissection. A spherical portion of dark red tissue measuring 0.2 × 0.1 × 0.1 cm was identified deep to the vascular insertion point. Along with routine sections, this portion of tissue was submitted for histologic examination.Histologic sections of the single shared placental disc (Fig 9) revealed multiple small septal cysts and villi appropriate for gestational age. The 3-vessel umbilical cord of the pump twin was unremarkable. Both an amnion and chorion were present within the membranes of the cardiac twin. No features of fetal distress (eg, meconium-laden macrophages) were identified on sections of the membranes. Histopathologic examination confirmed that the acardiac twin’s vessels were unprotected outside its amniotic sac; within the amniotic sac, edematous soft tissue was present surrounding the vessels. On sections of the membranes surrounding the acardiac twin, only the amnion was present.Tissue submitted from the acardiac twin revealed unremarkable skin, hair follicles, and subcutaneous fibroadipose tissue (Fig 10A). The small spherical portion of red tissue deep to the vascular insertion was identified histologically as the gastrointestinal tract (Fig 10B). The lumen showed microvilli, without meconium. No other organs were identified in routine histologic sampling.TRAP sequence is a complication of monochorionic twin pregnancies in which an anomalous twin with an absent or rudimentary heart (acardiac twin) is completely dependent on perfusion from its viable cotwin (pump twin) via aberrant arterioarterial placental anastomoses.Historically, TRAP sequence was thought to have an incidence of 1 in 35,000 pregnancies and approximately 1% of monochorionic gestations based on data obtained before 1953. However, a 2015 study presented evidence of a higher incidence of TRAP sequence—2.6% of monochorionic twin pregnancies and 1 in 9,500 to 11,000 pregnancies. (6) This difference was attributed to improved first-trimester ultrasound resolution and rising rates of multiple-gestation pregnancies with the increased use of assisted reproductive technologies. (6)The precise pathogenesis of TRAP sequence is unknown, but 2 pathways have been postulated. In 1 theory, the acardiac fetus receives deoxygenated blood via abnormal arterioarterial placental anastomoses formed in early embryogenesis, rather than oxygenated blood via direct placental perfusion; this deoxygenated blood results in compromised development of a functioning heart and other anatomic structures. In the alternative pathway, a chromosomal abnormality or environmental factors result in defective primary cardiac embryogenesis, thus creating low systemic pressure in the acardiac twin and retrograde blood flow from the pump twin. (3) In either case, circulatory failure occurs in the acardiac fetus at approximately 8 to 12 weeks’ gestation, resulting in abnormal morphogenesis. (7)In TRAP sequence, the pump twin has normal fetal circulation throughout gestation. However, anomalous arterioarterial placental anastomoses allow a portion of the pump twin’s cardiac output to travel retrograde through 1 or both umbilical arteries of the acardiac twin before entering its systemic circulation. This creates the