Ovarian hyperstimulation syndrome (OHSS)

Ovarian hyperstimulation syndrome (OHSS) is a complication of ovarian stimulation, most often related to gonadotropin injections and the trigger injection, in which the ovaries become enlarged and fluid can shift into the abdomen and, less commonly, the chest. Early OHSS usually appears within a few days of egg retrieval, while late OHSS can develop later, particularly if pregnancy occurs. Mild symptoms such as bloating and abdominal discomfort are common after stimulation, but OHSS ranges from mild to severe. Symptoms that need urgent attention include rapidly worsening abdominal swelling or pain, significant nausea or vomiting preventing fluids being kept down, reduced or no urination, breathlessness, calf swelling or pain, dizziness or fainting. Anyone experiencing these symptoms after fertility treatment should contact their treating clinic immediately, or go to the nearest hospital emergency department without delay if the clinic cannot be reached, as severe OHSS can become a medical emergency. Clinics reduce OHSS risk by tailoring stimulation medicine doses, choosing protocols such as the GnRH antagonist protocol, considering an agonist trigger where suitable, and by freezing all embryos for later transfer in some higher-risk cases. Anyone identified as higher risk, including women with PCOS or a high antral follicle count, should discuss their individual risk and monitoring plan with their treating clinic.

Also known as: OHSS, early OHSS, late OHSS, ovarian hyperstimulation

Last reviewed: 9 September 2026. Written by the Antega Editorial Team. General information only — always follow the advice of your own clinic, doctor or pharmacist.