Ovarian Hyperstimulation Syndrome (OHSS)

How OHSS develops

Fertility medicines stimulate the ovaries to grow many follicles. The final "trigger" injection, usually hCG, matures the eggs. hCG also causes the ovaries to release VEGF and other substances that make blood vessels leaky. Fluid moves out of the bloodstream into the abdomen (ascites) and sometimes around the lungs, while the blood becomes concentrated.

The result:

• swollen, enlarged ovaries
• fluid in the abdomen, causing bloating and weight gain
• low blood volume, which can reduce kidney blood flow
• thicker blood, which raises the risk of clots

If pregnancy occurs, rising hCG from the pregnancy can trigger or worsen late OHSS.

Risk factors

• polycystic ovary syndrome
• high antral follicle count or high AMH
• young age and low body weight
• previous OHSS
• many follicles (often more than 15 to 20) or high estradiol levels during stimulation
• hCG trigger and hCG used for luteal support
• pregnancy, especially multiple pregnancy

Severity

Symptoms

• abdominal bloating and fullness
• abdominal or pelvic pain
• nausea, vomiting, diarrhea
• rapid weight gain
• increasing waist size
• reduced urine output, dark urine
• shortness of breath, especially lying flat
• dizziness

Diagnosis

Diagnosis is based on timing after fertility treatment and symptoms. Your clinic may check:

• weight and abdominal girth daily
• ultrasound for ovarian size and fluid in the abdomen
• blood tests: hematocrit (blood concentration), electrolytes, kidney and liver function, clotting tests
• chest X-ray or ultrasound if breathing is affected
• pregnancy test

Treatment

Mild to moderate OHSS (usually at home)

• rest, but stay gently mobile to reduce clot risk
• drink to thirst, aiming for 2 to 3 liters a day including electrolyte drinks, unless advised otherwise
• record weight daily and urine output
• acetaminophen (paracetamol) for pain; avoid NSAIDs, which can affect the kidneys
• avoid intercourse and strenuous activity, which can cause ovarian twisting or rupture
• attend monitoring appointments

Severe OHSS (hospital)

• IV fluids and albumin to maintain circulation
• paracentesis: draining abdominal fluid with a needle to ease pain and breathing and improve kidney function, often done as an outpatient
• blood thinners (low-molecular-weight heparin) to prevent clots
• careful fluid balance monitoring
• intensive care for critical cases

OHSS usually resolves within 1 to 2 weeks, or by the end of the first trimester if pregnancy occurs.

Prevention

Fertility clinics use several strategies to reduce risk:

• individualized, lower gonadotropin doses
• GnRH antagonist protocols
• GnRH agonist trigger instead of hCG in high responders
• "freeze-all": freezing all embryos and transferring later, avoiding pregnancy hCG in the stimulated cycle
• cabergoline started at the trigger in high-risk patients
• metformin for some people with PCOS
• cancelling or coasting a cycle when response is excessive

Complications

• blood clots in the legs, lungs or, rarely, unusual sites such as the neck veins
• ovarian torsion
• kidney injury
• fluid around the lungs
• rupture of an ovarian cyst

Frequently asked questions

What are the symptoms of OHSS?

Mild OHSS causes bloating, abdominal discomfort and nausea. Severe OHSS causes rapid weight gain, a tense swollen abdomen, vomiting, shortness of breath and passing little urine. Contact your fertility clinic promptly if symptoms develop or worsen.

How long does OHSS last?

Early OHSS usually settles within 7 to 10 days if you do not become pregnant, often with your next period. If you are pregnant, symptoms can last longer and may worsen before gradually improving over a few weeks.

Can OHSS happen after egg retrieval?

Yes. OHSS typically develops within a few days after the trigger injection and egg retrieval. Late OHSS can appear 10 to 17 days after the trigger if pregnancy occurs, as pregnancy hormones stimulate the ovaries further.

Does OHSS affect pregnancy?

OHSS itself does not usually harm an ongoing pregnancy, but pregnancy can make OHSS more severe and longer-lasting. Many clinics freeze all embryos in high-risk cycles to avoid this, transferring them in a later cycle.