Ovarian hyperstimulation syndrome (OHSS) is the most serious potential complication of IVF stimulation. It occurs when the ovaries over-respond to FSH injections, producing too many follicles and causing fluid to leak from blood vessels into the abdomen and, in severe cases, the chest. For most patients it is mild and self-limiting; for a small number it requires hospitalisation.

Understanding your risk, how your clinic manages it, and what symptoms to watch for is an important part of IVF preparation — particularly if you have PCOS or PMOS, high AMH, or a history of strong stimulation response.

Who Is at Risk?

OHSS risk is primarily driven by ovarian reserve and sensitivity to FSH. The main risk factors:

  • High AMH or high antral follicle count (AFC): The strongest predictor. If your AMH is above 30–35 pmol/L or your AFC is above 20, your risk of moderate-to-severe OHSS with standard stimulation is meaningful.
  • PCOS/PMOS diagnosis: The polycystic ovarian morphology associated with PMOS creates a large pool of antral follicles, each of which may respond to FSH. See PCOS renamed PMOS: what it means for IVF patients.
  • Young age: Younger patients tend to have higher ovarian reserve and stronger responses.
  • Previous OHSS: A history of OHSS in a prior cycle significantly increases risk in subsequent cycles.
  • Low body weight: Lower body weight concentrates the same FSH dose, potentially leading to stronger response per kilogram.

Classification: Mild, Moderate, and Severe

Mild OHSS affects a significant proportion of stimulated cycles and typically causes:

  • Abdominal bloating and discomfort
  • Nausea
  • Slight ovarian enlargement

Mild OHSS does not require treatment beyond comfort measures and usually resolves within a week.

Moderate OHSS involves:

  • More pronounced abdominal distension
  • Nausea and vomiting
  • Detectable fluid accumulation (ascites) on ultrasound
  • Ovarian enlargement

Moderate cases are managed with close monitoring, hydration, and often a freeze-all strategy (no fresh transfer).

Severe OHSS is rare but can be dangerous:

  • Large-scale fluid accumulation in the abdomen and chest
  • Significant haemoconcentration (blood thickening), raising clotting risk
  • Kidney impairment in extreme cases
  • Hospitalisation required

Severe OHSS occurs in approximately 1–2% of IVF cycles overall, but risk rises substantially in patients with the risk factors listed above.

How Clinics Manage OHSS Risk

Tailored stimulation protocols. For high-risk patients, clinics use lower starting doses of FSH and monitor more frequently, adjusting the dose down if follicle numbers are exceeding target. The antagonist (GnRH antagonist) protocol is preferred over the long agonist protocol for PMOS patients, as it allows a GnRH agonist trigger (rather than hCG), dramatically reducing OHSS risk.

GnRH agonist trigger. The standard trigger for egg maturation is an hCG injection. In high-risk patients, substituting a GnRH agonist trigger (e.g., Lupron/buserelin) avoids the prolonged LH-like surge of hCG, which is the main driver of late-onset OHSS. This reduces severe OHSS risk substantially — but requires a freeze-all strategy (no fresh transfer in that cycle).

Freeze-all strategy. If stimulation produces a larger-than-expected response, freezing all embryos and cancelling the fresh transfer is often the safest approach. This eliminates the risk of late-onset OHSS (which is driven by rising hCG in early pregnancy) and gives the body time to recover before a frozen embryo transfer. See frozen embryo transfer guide.

Cabergoline. A dopamine agonist medication that can be prescribed after trigger to reduce fluid accumulation. Used in moderate-risk cases where a GnRH agonist trigger is not being used.

Cycle cancellation. If monitoring shows an excessive response before trigger, the clinic may cancel the cycle, avoid egg collection entirely, and plan a modified approach next time. This is a last resort but sometimes the safest option.

Symptoms to Watch For After Egg Collection

OHSS symptoms typically peak 3–5 days after egg collection (early OHSS) or 9–12 days after (late OHSS, linked to pregnancy). Contact your clinic if you experience:

  • Severe abdominal pain or bloating that is worsening
  • Nausea and vomiting preventing you from keeping fluids down
  • Reduced urine output (dark urine, urinating much less than usual)
  • Rapid weight gain (more than 1kg in 24 hours — fluid accumulation)
  • Difficulty breathing or shortness of breath
  • Dizziness or fainting

Frequently Asked Questions

Q: My AMH is very high. Does that mean I'll definitely get OHSS?

A: No. High AMH increases risk, but careful protocol design — lower FSH doses, antagonist protocol, GnRH agonist trigger — substantially reduces the probability of severe OHSS even in high-AMH patients. Many patients with AMH above 30 pmol/L complete IVF cycles without significant OHSS. The key is that your clinic recognises your risk profile and adapts accordingly.

Q: I had OHSS in a previous cycle. What should my next cycle look like?

A: Your previous cycle's response data is extremely useful for planning the next one. Lower starting dose, more frequent monitoring, GnRH antagonist protocol, and GnRH agonist trigger with freeze-all is a common approach. Discuss with your consultant specifically what was different about the previous cycle and how they would change the protocol.

Q: Can I have a fresh transfer if I have mild OHSS?

A: Mild OHSS does not automatically preclude fresh transfer. Moderate OHSS typically warrants freeze-all to avoid escalation to severe OHSS caused by early pregnancy hCG. Your clinic will assess each case individually based on ovarian size, symptom severity, and number of oocytes retrieved.

Q: Is OHSS more dangerous if I'm pregnant?

A: Yes. Pregnancy-related hCG drives late-onset OHSS, and in severe cases, the combination of OHSS and early pregnancy requires careful management. This is one of the key reasons freeze-all is recommended in high-risk cases — it allows the OHSS to resolve before attempting implantation.

Q: Does OHSS affect egg or embryo quality?

A: Mild-to-moderate OHSS does not directly damage the eggs collected. The concern is primarily patient safety and wellbeing, not embryo quality. However, the hormonal environment of an over-stimulated cycle may be less optimal for fresh embryo transfer, which is another reason freeze-all is often preferred.

This article is for information only and does not constitute medical advice. If you are experiencing symptoms of OHSS after egg collection, contact your clinic immediately.