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 donor twin becomes hypovolemic and oliguric oligohydramnios develops in the amniotic sac recipient twin becomes hypervolemic and polyuric polyhydramnios develops in the amniotic sac 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 smaller twin has oligohydramnios Treatment Procedural reduction amniocentesis drains amniotic fluid from around the recipient twin to ↓ polyhydramnios may improve circulation in the donor twin 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