Ovarian hyperstimulation syndrome (OHSS) is a complication of fertility treatment in which the ovaries respond too strongly to stimulation medication. The ovaries enlarge, and fluid leaks out of the blood vessels into the abdomen and, in severe cases, around the lungs.
Most cases are mild and settle on their own. A small number become serious enough to need hospital admission, which is why the condition is watched for carefully at every stage of a stimulated cycle. Modern treatment protocols have made severe OHSS far less common than it once was, and in most high-risk patients it can now be largely prevented.
What Happens in Ovarian Hyperstimulation Syndrome (OHSS)
The condition starts with the trigger injection given to complete the maturation of the eggs. In conventional protocols, this injection contains human chorionic gonadotropin (hCG). In an ovary with a large number of stimulated follicles, hCG causes the release of substances that make the walls of blood vessels more leaky.
Fluid then escapes from the circulation into the abdomen. As it does, the blood left behind becomes more concentrated, urine output falls, the kidneys come under strain, and the risk of blood clots rises. How severe the condition becomes depends on how much fluid shifts, not just on how large the ovaries are.
What Causes OHSS During IVF
OHSS during IVF appears in two distinct patterns, and the difference matters for monitoring.
Early OHSS: Appears within about a week of egg collection and is caused by the hCG trigger injection. It usually settles within days if pregnancy does not occur.
Late OHSS: Appears more than a week after collection and is caused by the hCG produced by an implanting pregnancy. It tends to last longer and is more often severe, because the hormone level keeps rising instead of falling.
This is why a patient at high risk may be advised to freeze all embryos and delay the transfer. If no pregnancy starts in the stimulated cycle, the late form of the condition cannot develop.
OHSS Symptoms
OHSS symptoms range from expected discomfort after egg collection to warning signs that need urgent attention. What matters most is whether the symptoms are getting better or worse.
Abdominal bloating and mild discomfort: Common after egg collection and usually settles within a few days.
Nausea and reduced appetite: Frequent in mild and moderate cases.
Rapid weight gain: Gaining more than about a kilogram in a day suggests fluid is accumulating and should be reported to the clinic.
Increasing abdominal swelling and pain: Discomfort that is getting worse rather than better indicates the condition is progressing.
Passing less urine: Noticeably reduced urine despite drinking fluids means the blood is becoming concentrated and needs prompt review.
Vomiting that prevents drinking: A reason to contact the clinic the same day.
Breathlessness or chest discomfort: May mean fluid has collected around the lungs or, rarely, a clot has formed. This needs immediate medical attention.
Pain or swelling in one calf: A possible sign of a clot in the leg, and treated as urgent.
Patients going through stimulation are advised to weigh themselves daily and to contact the clinic when symptoms worsen, rather than waiting for the next scheduled visit.
OHSS Risk Factors
OHSS risk factors are well known, which is what makes it possible to plan prevention before stimulation begins.
Polycystic ovary syndrome: Women with PCOS carry the highest risk of any group, because their ovaries contain a large number of small follicles capable of responding to stimulation.
High antral follicle count or high AMH: Both indicate an ovary primed to over-respond, even without a formal PCOS diagnosis.
Younger age: Ovarian reserve is generally higher at younger ages, so the response to stimulation is stronger.
Low body weight: Associated with a stronger response to a given dose of medication.
A previous episode of OHSS: One of the strongest predictors that it will happen again.
A fast-rising estradiol level or a large number of growing follicles during monitoring: Warning signs picked up during the cycle itself that prompt a change in the plan.
Pregnancy in the stimulated cycle: Raises the risk of the late form, particularly with twins.
OHSS Prevention
OHSS prevention has improved greatly, and the measures below have turned the condition from an accepted hazard of treatment into a largely avoidable one.
Individualized medication dosing: The starting dose of stimulation medication is set according to age, weight, AMH, and antral follicle count, and adjusted during the cycle based on the response.
Antagonist protocols: The GnRH antagonist protocol allows the trigger injection to be changed at short notice, and it is the preferred approach in patients at risk.
Agonist trigger instead of hCG: In an antagonist cycle, the eggs can be matured with a GnRH agonist trigger instead of hCG. The agonist produces a short natural hormone surge that matures the eggs while largely removing the cause of the condition.
Freeze-all strategy: All embryos are frozen, and the transfer is done in a later cycle. This prevents the late form of OHSS and gives the ovaries time to recover before pregnancy.
Cabergoline after egg collection: A short course of this medication reduces the leakiness of blood vessels and is given to at-risk patients.
Close monitoring: Ultrasound scans and estradiol blood tests during stimulation catch an excessive response early enough to lower the dose, change the trigger, or hold it back.
In vitro maturation as an alternative: For selected patients at the highest risk, IVM collects immature eggs with minimal or no stimulation, avoiding the condition at its source.
Canceling the cycle remains an option where the response is extreme, though with current protocols it is rarely needed.
Ovarian Hyperstimulation Syndrome Treatment
Ovarian hyperstimulation syndrome treatment depends on severity, and most cases are managed without admission.
Mild cases: Managed at home with plenty of oral fluids, adequate protein intake, simple painkillers (avoiding anti-inflammatory drugs unless advised), daily weight checks, and rest from strenuous activity. Symptoms usually settle within a week if pregnancy has not occurred.
Moderate cases: Reviewed in the clinic with an ultrasound and blood tests, closer tracking of weight and urine output, and anti-nausea medication where needed.
Severe cases: Admitted to hospital for intravenous fluids, correction of salt and mineral imbalances, medication to prevent clots, and drainage of the abdominal fluid where the swelling or breathing is significantly affected. Drainage relieves symptoms quickly and is a standard, well-tolerated procedure.
The condition resolves on its own. If pregnancy has not occurred, it settles as hCG falls. If pregnancy has occurred, it settles over a longer period under monitoring, while the pregnancy itself continues normally in the great majority of cases.
Conclusion
OHSS is the main medical complication of ovarian stimulation, and it is also one of the most predictable. The patients likely to develop it can be identified before treatment starts, and the measures that prevent it, from individualized dosing to the agonist trigger and the freeze-all approach, are standard practice in well-run programs.
For patients, two practical points matter. The risk should be discussed before stimulation starts, with the prevention plan explained. And worsening symptoms after egg collection should be reported to the clinic promptly rather than endured.
Consult a Fertility Specialist at 9M Fertility
The risk of OHSS can be assessed before treatment begins, and the protocol chosen at the start decides how well it is controlled.
The specialists at 9M Fertility review ovarian reserve, hormone levels, and previous treatment response before stimulation, select the protocol and trigger accordingly, and monitor each cycle closely so the plan can be adjusted where needed.
Book a consultation with 9M Fertility to discuss a treatment plan suited to your risk profile. Contact Us
