Ovarian hyperstimulation syndrome (OHSS) is a serious complication of assisted reproduction techniques using in vitro fertilization and intracytoplasmic sperm injection (IVF/ICSI). Its etiology has still not yet been fully resolved. Human chorionic gonadotrophin, administered exogenously as well as produced endogenously during pregnancy, is responsible for the onset of OHSS, and high levels of estradiol appear to worsen the condition. In this case series, the course of mild to severe OHSS was evaluated in 19 intact twin pregnancies after IVF/ICSI. Another serious complication associated with OHSS is thromboembolic events. In these cases, the pregnancy can be protected through anticoagulation treatment, but there may be exceptions to this. This series includes a case of bilateral thrombosis of the internal and external jugular veins in the 7th gestational week in a twin pregnancy after OHSS and ICSI, with termination of the pregnancy in the 9th gestational week due to progressive thrombosis during anticoagulation therapy.