Causes

The cause of ovarian hyperstimulation syndrome isn't fully understood. Having a high level of human chorionic gonadotropin (HCG) — a hormone usually produced during pregnancy — introduced into your system plays a role. Ovarian blood vessels react abnormally to HCG and begin to leak fluid. This fluid swells the ovaries, and sometimes large amounts move into the abdomen.

During fertility treatments, HCG may be given as a "trigger" so that a mature follicle will release its egg. OHSS usually happens within a week after you receive an HCG injection. If you become pregnant during a treatment cycle, OHSS may worsen as your body begins producing its own HCG in response to the pregnancy.

Injectable fertility medications are more likely to cause OHSS than is treatment with clomiphene, a medication given as a pill you take by mouth. Occasionally OHSS occurs spontaneously, not related to fertility treatments.