What Is Ovarian Hyperstimulation Syndrome?

Quick Answer

Ovarian hyperstimulation syndrome (OHSS) is a condition where the ovaries overreact to injectable fertility medications, causing them to swell and leak fluid into the abdomen. It is a potential complication of IVF and egg freezing cycles. Most cases are mild and resolve on their own, but severe OHSS is rare and requires medical attention. Modern protocols have significantly reduced the incidence of serious cases.

At Markham Fertility Centre, careful monitoring and individualized medication protocols are used to minimize OHSS risk for every patient.

How OHSS Develops

During IVF or egg freezing, injectable hormones (gonadotropins) stimulate the ovaries to produce multiple eggs. In some patients, the ovaries respond more aggressively than expected.

The key trigger is human chorionic gonadotropin (hCG), which is typically given as a “trigger shot” to mature the eggs before retrieval. In patients with an exaggerated ovarian response, hCG causes blood vessels in and around the ovaries to become leaky. This leads to:

• Swelling of the ovaries (sometimes to several times their normal size)
• Fluid shifting from blood vessels into the abdomen (ascites)
• In severe cases, fluid accumulation in the chest, blood clot risk, and electrolyte imbalances

OHSS symptoms typically appear 1 to 10 days after the hCG trigger shot. If pregnancy occurs, symptoms may worsen or last longer because the body produces its own hCG during early pregnancy.

Symptoms by Severity

Severity	Symptoms	Frequency
Mild	Bloating, mild abdominal discomfort, mild weight gain, nausea	Up to 33% of IVF cycles
Moderate	Increased abdominal swelling, vomiting, diarrhea, visible weight gain (>1 kg/day), reduced urination	3 to 8% of IVF cycles
Severe	Rapid weight gain, tense abdomen, difficulty breathing, blood clots, decreased urine output, severe pain	Less than 1% of IVF cycles

Mild OHSS is common and usually resolves within a few days without intervention. Severe OHSS is a medical emergency and may require hospitalization.

Who Is at Higher Risk

Certain patients are more likely to develop OHSS:

• PCOS (polycystic ovary syndrome): The strongest risk factor. PCOS patients have a higher number of follicles and are more sensitive to stimulation medications.
• High AMH levels: AMH above 3.5 ng/mL indicates high ovarian reserve and increased sensitivity to gonadotropins
• High antral follicle count (AFC): More than 24 follicles on baseline ultrasound
• Young age: Patients under 30 are at higher risk due to greater ovarian reserve
• Low body weight: BMI under 18.5 is associated with increased OHSS risk
• Previous OHSS: Having had OHSS in a prior cycle significantly increases the risk of recurrence
• High estradiol levels during stimulation: Rapidly rising or very high estradiol (above 3,500 pg/mL) before the trigger shot

Your fertility physician assesses these risk factors before and during every stimulation cycle.

How OHSS Is Prevented

Modern IVF protocols have made severe OHSS rare. Prevention strategies include:

• Individualized medication dosing: Lower starting doses for high-risk patients (PCOS, high AMH, low BMI) with careful dose adjustments based on monitoring
• GnRH agonist trigger (Lupron trigger): Replaces the traditional hCG trigger shot in high-risk patients. Dramatically reduces OHSS incidence because GnRH agonist clears from the body faster than hCG.
• Freeze-all strategy: Instead of transferring a fresh embryo, all embryos are frozen and the transfer is postponed to a later cycle. This avoids the additional hCG surge from early pregnancy that worsens OHSS.
• Coasting: Temporarily withholding stimulation medications when estradiol levels rise too quickly, allowing levels to stabilize before triggering
• Careful monitoring: Frequent blood work (estradiol) and ultrasounds during stimulation to track follicle count and hormone levels. The physician can adjust or cancel a cycle if the response is too aggressive.
• Cabergoline (Dostinex): A dopamine agonist that can be prescribed after trigger to reduce fluid leakage from the ovaries

At Markham Fertility Centre, monitoring appointments are available across five locations (Markham, Barrie-Innisfil, Oshawa, Orillia, Sudbury), making it easier for patients to attend the frequent visits that careful OHSS prevention requires.

What to Do If You Develop Symptoms

For mild symptoms:

• Rest and avoid strenuous activity
• Drink fluids regularly (electrolyte drinks can help)
• Avoid alcohol and caffeine
• Weigh yourself daily and track any rapid changes
• Contact your clinic if symptoms worsen

Seek immediate medical attention if you experience:

• Rapid weight gain (more than 1 kg per day)
• Severe abdominal pain or tense, distended abdomen
• Difficulty breathing or shortness of breath
• Decreased urination (significantly less than normal)
• Dizziness or fainting

Severe OHSS may require IV fluids, drainage of abdominal fluid (paracentesis), and in rare cases, blood thinners to prevent clots.

OHSS and Egg Retrieval Comfort

OHSS risk is one reason why the egg retrieval experience matters. Patients with OHSS may have enlarged, sensitive ovaries that make retrieval more uncomfortable.

At Markham Fertility Centre, every egg retrieval is performed with a dedicated anesthesiologist, ensuring a pain-free procedure regardless of ovarian size or sensitivity. This is especially important for PCOS patients and others at elevated OHSS risk, where the ovaries may be significantly enlarged at the time of retrieval.

FAQ

How common is OHSS during IVF?

Mild OHSS affects up to one-third of IVF patients. Moderate OHSS occurs in 3 to 8% of cycles. Severe OHSS is rare, affecting less than 1% of patients with modern prevention protocols.

Can OHSS be completely prevented?

The risk can be significantly reduced but not eliminated entirely. GnRH agonist triggers, freeze-all strategies, and individualized dosing have made severe OHSS very rare at experienced clinics.

Does OHSS affect egg quality or IVF success?

No. OHSS does not damage the eggs. Patients who develop OHSS and freeze their embryos have the same IVF success rates when they return for a frozen embryo transfer.

How long does OHSS last?

Mild OHSS typically resolves within a few days to a week. If pregnancy occurs in the same cycle, symptoms may persist for 2 to 3 weeks as hCG levels rise. With a freeze-all approach, symptoms resolve faster because there is no pregnancy-related hCG.

Am I at risk if I have PCOS?

Yes. PCOS is the strongest risk factor for OHSS because of the high number of follicles and heightened sensitivity to stimulation medications. Your physician will adjust your protocol accordingly, often using lower medication doses and a GnRH agonist trigger.

Can OHSS happen with IUI?

Rarely. OHSS is primarily associated with injectable gonadotropins used in IVF. IUI cycles using oral medications (letrozole or Clomid) carry a very low OHSS risk. However, if injectable medications are used for IUI and too many follicles develop, the cycle may be cancelled to prevent OHSS and the risk of high-order multiple pregnancy.