OHSS happens when the ovaries respond too strongly to the hormone injections used in IVF or egg freezing. The enlarged ovaries release fluid into the abdominal cavity, which produces bloating, abdominal pain and a swollen waistline; in its more severe forms it causes dehydration, breathlessness, reduced urine output and a raised risk of blood clots. Mild OHSS is common and settles on its own. Moderate to severe OHSS is uncommon and needs medical assessment.
Who is at risk, and how it's prevented
The strongest responders carry the highest risk: a high AMH, a large antral follicle count, PCOS, a young age, or a previous episode. Prevention is built into how a cycle is run — the starting dose is matched to your ovarian reserve, monitoring scans and estradiol levels catch an over-response while the plan can still change, and where the response looks strong the hCG trigger is often replaced with a GnRH-agonist trigger. The more dangerous late-onset form is driven by the hCG of an early pregnancy, so freezing all embryos instead of transferring in the same cycle removes that pathway. Egg freezing side effects and risks covers the warning signs in full.