OHSS (ovarian hyperstimulation syndrome) is what happens when the ovaries respond too strongly to the hormone injections used in IVF or egg freezing. The enlarged ovaries leak fluid into the abdomen (belly). This causes bloating, abdominal pain and a swollen waistline. In more severe forms it causes dehydration, breathlessness, passing less urine and a higher risk of blood clots. Mild OHSS is common and settles on its own within a week or two. Moderate to severe OHSS is uncommon and needs medical assessment.
Warning signs — when to call your clinic
Mild bloating and heaviness toward the end of stimulation are expected. Call the same day if you notice:
- Rapid or severe abdominal swelling, rather than a gradual increase
- Pain that's worsening rather than steady
- Breathlessness, even mild
- Little or no urine output over several hours
- Persistent vomiting or an inability to keep fluids down
Monitoring visits during ovarian stimulation and egg retrieval are designed to catch these signs early. That is why the visits don't stop once the injections are done.
Who is at risk, and how it's prevented
Women whose ovaries respond most strongly carry the highest risk. Signs of this are a high AMH, a large antral follicle count (many small follicles on the scan), 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.
- 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 hormone of an early pregnancy. So freezing all embryos, instead of transferring one in the same cycle, removes that cause. Egg freezing side effects and risks covers the warning signs in the egg-freezing context specifically.