What causes ohss

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Last updated: April 4, 2026

Quick Answer: Ovarian Hyperstimulation Syndrome (OHSS) is typically caused by the medications used to stimulate egg production during fertility treatments, particularly injectable forms of human chorionic gonadotropin (hCG). These medications can cause the ovaries to become enlarged and fluid to leak into the abdominal cavity.

Key Facts

OHSS is a complication of fertility treatments, affecting approximately 3-8% of women undergoing IVF.
The primary trigger for OHSS is the hormone hCG, often administered to induce final egg maturation.
Risk factors include a history of OHSS, polycystic ovary syndrome (PCOS), and a high number of developing follicles.
Mild OHSS symptoms can resolve on their own within a week, while severe cases may require hospitalization.
The syndrome involves the ovaries becoming enlarged and fluid accumulating in the abdomen.

What Causes Ovarian Hyperstimulation Syndrome (OHSS)?

Ovarian Hyperstimulation Syndrome (OHSS) is a medical condition that can arise as a complication of fertility treatments. It is characterized by the ovaries becoming abnormally enlarged and potentially leaking fluid into the abdominal cavity. While the exact mechanisms are complex, the primary cause is the response of the ovaries to certain fertility medications, particularly those used in In Vitro Fertilization (IVF) protocols.

The Role of Fertility Medications

The core of OHSS development lies in the ovarian response to the medications designed to stimulate the development of multiple eggs. These medications often include follicle-stimulating hormone (FSH) and luteinizing hormone (LH) or analogues thereof. The goal is to encourage the ovaries to produce a significant number of mature eggs for retrieval. However, in some individuals, the ovaries can overreact to these medications, leading to excessive follicle growth and an increase in hormone levels, specifically estrogen.

The hCG Trigger

A critical factor in the onset of OHSS is often the administration of human chorionic gonadotropin (hCG). This hormone is typically given as a "trigger shot" to induce the final maturation of the eggs before they are retrieved for IVF. hCG mimics the natural LH surge that occurs during a spontaneous ovulation cycle. However, when the ovaries have already been hyperstimulated by FSH/LH medications, the administration of hCG can exacerbate the condition significantly. It can lead to a surge in vascular permeability, causing fluid to shift from the bloodstream into the abdominal cavity and other body spaces.

Mechanisms of OHSS Development

The development of OHSS involves several physiological changes:

Ovarian Enlargement: The ovaries become significantly enlarged due to the proliferation of numerous follicles. These follicles are fluid-filled sacs that contain the developing eggs.
Increased Vascular Permeability: The body releases certain substances, like vascular endothelial growth factor (VEGF), in response to the hormonal stimulation. VEGF increases the permeability of blood vessels, allowing fluid and proteins to leak out of the capillaries.
Fluid Accumulation: This leaked fluid accumulates in the peritoneal cavity (abdomen), leading to bloating and abdominal distension. In severe cases, fluid can also collect in the chest cavity (pleural effusion) or around the heart (pericardial effusion).
Electrolyte Imbalance: The fluid shift can lead to dehydration and imbalances in electrolytes like sodium and potassium, which are crucial for bodily functions.
Thromboembolism Risk: Severe OHSS is associated with an increased risk of blood clots due to the concentration of the blood resulting from fluid loss.

Risk Factors for OHSS

While OHSS can occur in any woman undergoing ovarian stimulation, certain factors increase the risk:

Young Age: Women under 35 years old tend to have a higher risk.
Polycystic Ovary Syndrome (PCOS): Women with PCOS often have a greater number of small follicles, making their ovaries more sensitive to stimulation.
Previous History of OHSS: If a woman has experienced OHSS before, her risk of recurrence is higher.
High Number of Follicles: A large number of developing follicles, typically more than 20-25, is a strong predictor of OHSS.
High Estrogen Levels: Elevated levels of estrogen, particularly before the hCG trigger shot, can indicate an increased risk.
Underlying Medical Conditions: Certain conditions might influence susceptibility.

Types and Severity of OHSS

OHSS is generally classified into three categories based on its severity:

Mild OHSS: This is the most common form and typically involves mild abdominal discomfort, bloating, nausea, and slight ovarian enlargement (up to 5 cm). Symptoms usually resolve within a week.
Moderate OHSS: Symptoms are more pronounced, including significant abdominal pain, vomiting, diarrhea, and moderate ovarian enlargement (5-12 cm). Fluid accumulation may be noticeable.
Severe OHSS: This is a rare but serious form that can lead to rapid weight gain, severe abdominal distension, shortness of breath, decreased urine output, blood clots, and kidney failure. Ovarian enlargement can exceed 12 cm, and significant fluid accumulation in the abdomen and chest is common. Severe OHSS often requires hospitalization for monitoring and treatment.

Understanding the causes and risk factors for OHSS is crucial for fertility specialists to implement preventative strategies and manage the condition effectively, ensuring the safety and well-being of patients undergoing fertility treatments.