Ovarian hyperstimulation syndrome, abbreviated OHSS, is an atypical reaction that women may experience in response to excessive hormones, and often occurs during fertility treatments. OHSS is typically triggered by hormonal medications designed to mature eggs in the ovaries, which can cause blood vessels within the ovaries to leak fluid. Sometimes that can lead to painful tenderness or swelling. In severe cases of OHSS, that fluid can leak into the abdominal cavity in large amounts, causing vomiting, blood clots, and severe pain. As many as one out of three women undergoing fertility treatment will experience some form of OHSS, although more severe presentations are rare. While the exact cause of OHSS is not fully understood as of 2020, researchers continue to discover various risk factors, prevention techniques, and treatments that may lead to decreased risks associated with OHSS and better fertility outcomes.