A 31-year-old woman, gravida 2 para 1, at 10 weeks gestation comes to the office for an initial prenatal visit. The patient has had no vaginal bleeding or cramping. Her first pregnancy was uncomplicated and ended with a term spontaneous vaginal delivery. She has no chronic medical conditions and has had no previous surgeries. The patient takes a daily prenatal vitamin and does not use tobacco, alcohol, or illicit drugs. Blood pressure is 120/70 mm Hg and pulse is 68/min. BMI is 24 kg/m². The uterine fundus is palpated above the pubic symphysis. Pelvic ultrasound shows 2 viable intrauterine gestations, a single fundal placenta, and a thin intertwin membrane that meets the placenta at a 90-degree angle. This patient is at highest risk for which of the following complications?

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11ec25c8_da4c_40ae_bc81_3fac93d045c4_MD0003_00 Twin gestations are at increased risk for many pregnancy complications. This risk is further stratified based on the chorionicity (number of placentas) and amnionicity (number of amniotic sacs) of the gestation. This patient has monochorionic diamniotic twins (1 placenta, 2 amniotic sacs) based on 2 embryos, a single placenta, and a thin intertwin membrane (composed of 2 amniotic sacs) that meets the placenta at a 90-degree angle ("T sign"). In patients who appear to have a single placenta, the base shape of the intertwin membrane distinguishes between a monochorionic ("T sign") and fused dichorionic ("lambda sign") gestation. "T sign" "lambda sign" Monochorionic twins are at risk for twin-twin transfusion syndrome (TTTS), a complication that can result in heart failure and fetal/neonatal mortality in both twins. In TTTS, unbalanced arteriovenous anastomoses are present between the shared placental vessels that supply the twins. Because of these anastomoses, blood from the placental arteries (high resistance/pressure) of one twin (donor) is shunted into the placental veins (low resistance/pressure) of the other twin (recipient). The shunting of blood away from the donor twin causes anemia that leads to renal failure, oligohydramnios, low-output heart failure, and fetal growth restriction. In contrast, the shunting of blood toward the recipient twin causes polycythemia, which leads to polyhydramnios, cardiomegaly, high-output heart failure, and hydrops fetalis. Both twins are at risk for intrauterine and neonatal death. Mild TTTS is expectantly managed with serial ultrasounds to evaluate for worsening clinical features. Moderate-to-severe cases are treated with laser coagulation of the placental anastomoses. (Choices A and B) In monozygotic twins, placentation type is determined by timing of the twinning. Twinning that occurs shortly after fertilization yields a dichorionic diamniotic gestation. In contrast, the incomplete division (ie, fission) that can lead to conjoined twins occurs later in development and yields a monochorionic monoamniotic gestation. Because the twins are in the same sac, monochorionic monoamniotic gestations can be complicated by cord entanglement. This patient has diamniotic twins. cord entanglement (Choice C) Risk factors for placenta accreta, implantation of the placenta directly into the myometrium, include placenta previa and prior uterine surgery (eg, cesarean delivery, myomectomy). (Choice D) Twins are at increased risk of placenta previa (placental tissue that covers the internal cervical os); however, this patient has a fundal placenta, making this complication unlikely. Educational objective: Monochorionic twin gestations can be complicated by twin-twin transfusion syndrome, a potentially fatal condition that results from unbalanced vascular anastomoses between the vessels supplying the umbilical cords of each twin. References: A systematic approach to the differential diagnosis and management of the complications of monochorionic twin pregnancies. (http://www.ncbi.nlm.nih.gov/pubmed/23006773) Diagnosis and management of twin-twin transfusion syndrome. (http://www.ncbi.nlm.nih.gov/pubmed/26165181) Twin-twin transfusion syndrome-what we have learned from clinical trials. (http://www.ncbi.nlm.nih.gov/pubmed/29122542) First-trimester ultrasound determination of chorionicity in twin pregnancy. (http://www.ncbi.nlm.nih.gov/pubmed/21308842)

A 31-Year-Old Woman, Gravida 2 Para 1, at 10 Weeks