Ovarian Hyperstimulation Syndrome (OHSS): What Every Fertility Patient Should Know

A complication of IVF that is more common than many patients realise — but also one that modern fertility care is getting much better at preventing.

What Is OHSS?

Ovarian Hyperstimulation Syndrome (OHSS) is one of the most significant short-term complications of fertility treatment. It occurs when the ovaries overreact to the hormone injections used during IVF and other assisted reproduction procedures to stimulate egg production.

In a normal IVF cycle, the goal is to encourage the ovaries to produce multiple follicles. In

some women, however, the response becomes excessive. The ovaries can become enlarged and begin leaking fluid into the abdominal cavity — and in serious cases, into the chest. This fluid shift causes a cascade of problems including bloating, pain, dehydration, and in severe cases, blood clots or organ complications.

As the Royal College of Obstetricians and Gynaecologists (RCOG) notes in its updated Green-top Guideline No. 5, OHSS is associated with significant physical and psychological morbidity and, in rare cases, has been linked to maternal death. The good news is that in most cases it is self-limiting — it resolves on its own with careful monitoring and supportive care.

Who Is at Risk?

Not everyone undergoing fertility treatment is equally vulnerable to OHSS. Certain factors significantly raise the risk, and identifying these early is central to safe practice.

The British Fertility Society (BFS) published updated guidelines in December 2024 specifically focused on prevention, highlighting the following high-risk groups:

• Women with Polycystic Ovary Syndrome (PCOS) — by far the most significant risk factor

• Women with a high antral follicle count (AFC >24) or an elevated AMH (>22.5 pmol/mL), indicating a high ovarian reserve

• Younger women and those with a low BMI

• Women who have previously experienced OHSS

• Those who produce a high number of eggs in a stimulation cycle

  
  

During treatment itself, the appearance of many developing follicles on ultrasound scan are important warning signs that risk is escalating.

Prevention: Getting Ahead of the Problem

Prevention is where the most significant advances have been made in recent years. The BFS guidelines make clear that early identification of risk, combined with careful protocol tailoring, can substantially reduce the incidence and severity of OHSS.

Key preventive strategies include:

GnRH Antagonist Protocols — For women at elevated risk, using a GnRH antagonist protocol (rather than the older "long protocol") is now firmly recommended. It allows a safer method of triggering egg maturation.

GnRH Agonist Trigger — Instead of the standard hCG injection to trigger final egg maturation, using a GnRH agonist trigger dramatically lowers OHSS risk. This is one of the most effective tools available.

Elective Freeze-All Strategy — Freezing all embryos rather than transferring them fresh removes the additional hormonal stimulus of early pregnancy, which can worsen OHSS significantly. A frozen embryo transfer in a subsequent cycle carries no additional OHSS risk.

Cabergoline — A dopamine agonist shown to reduce the severity of OHSS by blocking the vascular leakage that drives the condition. It is widely used as a preventive adjunct in high-risk patients.

Careful Gonadotropin Dosing — Starting with lower doses and adjusting based on response helps avoid over-stimulation from the outset.

Management: When OHSS Does Occur

The RCOG classifies OHSS into four grades — mild, moderate, severe, and critical — with management tailored to each.

Mild OHSS — Experienced by a large proportion of stimulated patients, this typically involves some bloating and pelvic discomfort. It can almost always be managed at home with rest, adequate fluid intake, and paracetamol. NSAIDs such as ibuprofen should be avoided as they may worsen kidney function.

Moderate OHSS — Ultrasound confirms ascites (fluid in the abdomen) alongside more pronounced symptoms. Close outpatient monitoring — including blood tests to check haematocrit, electrolytes and kidney function — is essential.

Severe and Critical OHSS — These cases require hospital admission. Management focuses on maintaining fluid balance (often with IV fluids or albumin), thromboprophylaxis to prevent blood clots, drainage of significant ascitic fluid (paracentesis), and careful monitoring of organ function. The RCOG guideline is clear: early recognition and prompt assessment are the cornerstones of safe management.

The Takeaway

OHSS is a serious but largely preventable complication of fertility treatment. Modern protocols, careful patient selection, and individualised treatment plans have transformed how we approach it. If you are about to begin IVF or ovarian stimulation, your fertility specialist should discuss your personal risk with you and explain what measures will be in place to protect you.

References: RCOG Green-top Guideline No. 5 — The Management of Ovarian Hyperstimulation Syndrome (updated April 2026); British Fertility Society Policy and Practice Guideline — Prevention of OHSS (December 2024).