Snapshot A 24-year-old G2P0 woman presents to the obstetrician at 25 weeks gestation. She complains that her abdomen seems to have rapidly enlarged over the past 2 weeks. An ultrasound is performed and is notable for polyhydramnios in 1 sac, explaining her enlarged abdomen, and oligohydramnios in the other sac, causing it to be stuck to the anterior uterine wall (see image). Introduction Overview twin-to-twin transfusion syndrome results when there is intrauterine blood transfusion from 1 twin (donor) to the other twin (recipient) Epidemiology Incidence 3-5 per 1000 pregnancies ETIOLOGY Pathophysiology result of transfusion of blood from 1 fetal twin to another twin through placental vascular anastomoses hypoperfusion of the donor twin and hyperperfusion of the recipient twin login to view 4 more bullets specific complication of monozygotic twins with monochorionic placentation may be monoamniotic or diamniotic Imaging Ultrasound indications diagnosis of twin-to-twin transfusion syndrome findings significant discrepancy in size of same-sex fetuses monochorionic placentation disparity in amount of amniotic fluid between the fetuses login to view 1 more bullet Treatment Procedural reduction amniocentesis drains amniotic fluid from around the recipient twin to ↓ polyhydramnios login to view 1 more bullet laser photocoagulation of anastomotic vessels reserved for more severe cases that do not respond to amnioreduction decreases shunting of blood from one fetus to the other