Ovarian hyperstimulation syndrome (OHSS) is a reaction to fertility medications where the ovaries swell and leak fluid into the body. It occurs when the ovaries are overstimulated during treatments like IVF, producing too many egg follicles at once. Mild OHSS affects up to 33% of women undergoing IVF, though severe cases are rare, occurring in roughly 1% of cycles.
What Causes OHSS
During IVF or other fertility treatments, injectable hormones encourage the ovaries to develop multiple egg follicles instead of the single follicle that would normally mature each month. In some women, the ovaries overreact. The trigger shot given to finalize egg maturation (a hormone called hCG) plays a central role: it causes a spike in a protein that makes blood vessel walls more permeable. Specifically, this protein loosens the connections between the cells lining blood vessels, creating gaps that allow fluid to leak out of the bloodstream and pool in the abdomen, and sometimes around the lungs.
The result is a paradox. Fluid builds up outside the blood vessels (causing bloating and swelling), while the bloodstream itself loses volume. This means the blood becomes thicker and more concentrated, which raises the risk of clotting and puts strain on the kidneys.
Who Is Most at Risk
Certain women are significantly more likely to develop OHSS. The strongest predictors include being under 35, having a low body weight, and having polycystic ovary syndrome (PCOS). Women with high ovarian reserve, meaning the ovaries contain a large number of immature eggs, face elevated risk. A blood marker called AMH helps estimate this reserve. In one study, women with AMH levels above 3.36 ng/ml had a dramatically higher chance of OHSS: 29% of women in the highest AMH group developed it, compared to 0% in the lowest group.
The number of follicles developing during a treatment cycle matters too. Having 11 or more follicles measuring at least 10 mm on the day of the trigger shot, or estradiol levels above 4,000 pg/ml, both signal increased risk. Fertility specialists monitor these numbers closely during stimulation to decide whether to adjust the protocol.
Symptoms by Severity
OHSS is graded from mild to critical, and the difference between grades is significant.
- Mild (Grades 1–2): Abdominal bloating, discomfort, nausea, vomiting, or diarrhea. The ovaries enlarge to between 5 and 12 cm. Most women with OHSS fall into this category, and symptoms typically resolve on their own within a week or two.
- Moderate (Grade 3): The same symptoms as mild OHSS, plus visible fluid accumulation in the abdomen (ascites) detectable on ultrasound.
- Severe (Grades 4–5): Significant abdominal fluid, possible fluid around the lungs causing breathing difficulty, thickened blood, and reduced kidney function. At this stage, hospitalization is usually necessary.
- Critical: Kidney failure, liver damage, blood clots, acute respiratory distress, or multiorgan failure. This requires intensive care and is the rarest form.
Early Onset vs. Late Onset
OHSS can appear in two distinct windows. Early-onset OHSS develops within the first nine days after the trigger shot and results from the ovaries’ excessive response to the stimulation medications themselves. Late-onset OHSS appears later and is closely tied to pregnancy. If an embryo implants, the body begins producing its own hCG, which can reignite or worsen the syndrome. This is why late-onset OHSS tends to be more persistent: the pregnancy sustains the hormonal signal driving it.
Serious Complications
Among women hospitalized with severe OHSS, life-threatening complications occur in about 4.4% of cases. The most common is blood clots (deep vein thrombosis or pulmonary embolism), affecting roughly 2.2%. Acute kidney failure and respiratory distress each occur in under 1% of severe cases. Women who have preexisting health conditions face roughly double the odds of developing blood clots or kidney failure compared to otherwise healthy patients.
These complications arise from the same underlying mechanism: fluid leaving the bloodstream makes the remaining blood thicker and more prone to clotting, while reduced blood volume means the kidneys receive less flow and can begin to shut down.
How OHSS Is Prevented
Modern fertility medicine has several tools to reduce OHSS risk, and prevention strategies have improved substantially over the past decade. The most effective approach for high-risk women involves two changes to the standard IVF protocol. First, instead of using hCG as the trigger shot, doctors can use a different type of trigger (a GnRH agonist) that matures the eggs without flooding the body with the hormone that drives OHSS. Second, rather than transferring a fresh embryo, all embryos are frozen for transfer in a later cycle. In a study of 123 women treated this way, zero cases of OHSS occurred, either early or late onset, and pregnancy rates remained strong at about 50% after two frozen embryo transfers.
Other prevention measures include “coasting,” which means pausing stimulation medications for a day or two to let hormone levels drop before giving the trigger. A medication that blocks a specific hormone receptor can also be given for eight days starting on trigger day to reduce the vascular leakiness that causes fluid shifts.
Managing Mild OHSS at Home
Most cases of OHSS are mild and can be managed without hospitalization. The key priorities are staying hydrated, tracking your symptoms, and knowing when things are getting worse. Practical steps include:
- Drink electrolyte-rich fluids like sports drinks to help maintain blood volume. Water alone isn’t ideal because it doesn’t replace the salts your body is losing.
- Weigh yourself daily. A gain of more than 2 pounds in a single day suggests fluid is accumulating rapidly and warrants a call to your clinic.
- Avoid vigorous physical activity. Enlarged ovaries are at risk of twisting (ovarian torsion), which is a surgical emergency.
- Use acetaminophen for pain. Avoid anti-inflammatory painkillers like ibuprofen, which can affect kidney function.
- Watch for escalation. Increasing abdominal size, difficulty breathing, reduced urination, or dizziness are signs that mild OHSS may be progressing.
Mild symptoms generally peak about five to seven days after the trigger shot and improve over the following week. If you become pregnant in the same cycle, symptoms can linger or worsen for several more weeks as pregnancy hormones rise.
Hospital Treatment for Severe Cases
When OHSS becomes severe, the main goals of treatment are restoring blood volume, relieving pressure from accumulated fluid, and preventing clotting. Intravenous fluids help counteract the dehydration caused by fluid leaking out of the bloodstream. If fluid buildup in the abdomen becomes severe enough to compress the lungs or cause significant pain, doctors can drain it using a needle guided by ultrasound. Blood thickness is monitored closely, and blood-thinning medication may be given to prevent clots.
Most women with severe OHSS recover fully within one to two weeks, though late-onset cases tied to pregnancy can take longer. The syndrome does not typically affect the pregnancy itself once it resolves.