Ovarian hyperstimulation syndrome (OHSS) is a potentially serious complication of IVF in which the ovaries overreact to fertility medications, causing fluid to leak from blood vessels into the abdomen and other body cavities. Moderate to severe cases occur in roughly 1% to 5% of IVF cycles. Most cases are mild and resolve on their own, but severe OHSS can require hospitalization and, rarely, become life-threatening.
What Happens in Your Body
During IVF, injectable hormones stimulate the ovaries to produce multiple eggs at once. In some women, the ovaries respond too aggressively. When a trigger shot (typically hCG) is given to mature the eggs before retrieval, it sends a signal that ramps up production of a protein called vascular endothelial growth factor, or VEGF. VEGF peaks about 48 hours after the trigger and loosens the tiny junctions between the cells lining your blood vessels. The result is that fluid seeps out of the bloodstream and collects where it shouldn’t, primarily in the abdominal cavity.
The ovaries themselves are the main source of this VEGF surge. As they swell with stimulated follicles, they release VEGF and other inflammatory signals that make capillaries throughout the body more permeable. This fluid shift is the core problem in OHSS: your blood volume drops while fluid accumulates in your belly, and sometimes around the lungs. That imbalance drives most of the symptoms.
Early vs. Late Onset
OHSS comes in two timing patterns. Early-onset OHSS appears 3 to 7 days after the hCG trigger shot and is usually mild to moderate. It’s driven by the trigger medication itself. Late-onset OHSS shows up 12 to 17 days after the trigger, tends to be more severe, and is fueled by hCG produced by an implanting pregnancy. This is why becoming pregnant during the same cycle as egg retrieval can worsen or prolong the condition.
Symptoms by Severity
Mild OHSS is common and often goes unnoticed or feels like routine post-retrieval discomfort. You might have bloating, mild abdominal pain, and slight nausea. Your ovaries may be moderately enlarged.
Moderate OHSS brings more noticeable abdominal swelling as fluid begins to accumulate. Nausea can worsen, and you may experience vomiting or diarrhea. The bloating becomes uncomfortable enough that pants don’t fit and it’s hard to eat full meals.
Severe OHSS is a medical emergency. The abdomen becomes visibly distended with fluid, and breathing can become difficult if fluid collects around the lungs. Rapid weight gain (several pounds in a day or two) is a hallmark. The drop in blood volume can lead to dangerous complications: blood clots, kidney problems from reduced blood flow, and electrolyte imbalances. Although rare, severe OHSS requires close monitoring and sometimes hospitalization.
Who Is Most at Risk
Certain factors make OHSS significantly more likely. The strongest predictors are markers of high ovarian reserve, meaning the ovaries have a large pool of follicles ready to respond to stimulation.
- High antral follicle count (AFC): An AFC of 16 or higher on ultrasound signals elevated risk. For women 35 and older, the threshold drops to about 15.
- Elevated AMH levels: An anti-Müllerian hormone level at or above 4.38 ng/mL is associated with an exaggerated response. For women under 35, the cutoff is slightly higher at about 4.95 ng/mL.
- Polycystic ovary syndrome (PCOS): Women with PCOS typically have high follicle counts and AMH levels, placing them squarely in the high-risk category.
- Young age: Younger women tend to have more responsive ovaries.
- Low body weight: A smaller body means less volume to buffer the fluid shifts.
- Previous OHSS: If it happened in an earlier cycle, the risk is higher in subsequent ones.
The type and dose of stimulation medications, along with the trigger strategy, also play a major role.
How OHSS Is Prevented
The single most effective prevention strategy involves changing the trigger medication. The traditional hCG trigger has a long-lasting biological effect that sustains the VEGF surge for days. Switching to a GnRH agonist trigger produces a shorter, more controlled hormonal peak that dramatically reduces OHSS risk. In one study comparing the two approaches in high responders, zero cases of OHSS occurred in the GnRH agonist-only group, while seven patients developed it in the group that received some hCG.
A “dual trigger” that combines a small dose of hCG with a GnRH agonist is sometimes used as a middle ground. It lowers the OHSS risk compared to a full hCG trigger while still providing some direct hormonal support for egg maturation. For patients at highest risk, though, the agonist-only trigger is preferred because it produces comparable long-term pregnancy rates with virtually no OHSS risk.
Other prevention strategies your clinic may use include lowering the dose of stimulation medications, “coasting” (pausing medication for a day or two before the trigger to let hormone levels drop), and freezing all embryos instead of doing a fresh transfer. Freezing avoids the possibility of pregnancy-driven hCG worsening OHSS in the weeks after retrieval.
What Treatment Looks Like
Mild cases are managed at home. You’ll be advised to stay hydrated (but not overhydrate), eat salty foods, avoid strenuous activity, and track your weight daily. Weight gain of more than two pounds in a single day, worsening bloating, or difficulty breathing are signs to call your clinic immediately.
Moderate cases may require more frequent clinic visits for blood work and ultrasounds to monitor fluid levels and kidney function. Pain management with over-the-counter medication and rest are typically sufficient.
Severe cases often require hospitalization. The most common procedure is paracentesis, where a needle is used to drain accumulated fluid from the abdomen. This can provide rapid relief from pressure and breathing difficulty, and it may need to be repeated. Intravenous fluids help restore blood volume, and blood-thinning medication may be given to prevent clots. In critical situations, doctors monitor for kidney function changes and respiratory distress.
How Long It Lasts
If you don’t become pregnant in the same cycle, OHSS typically resolves within 7 to 10 days as hormone levels fall with your next period. If you do become pregnant, rising hCG from the pregnancy can sustain or worsen symptoms for several weeks, sometimes into the first trimester. This is one reason many clinics now recommend a “freeze-all” strategy for patients showing signs of OHSS: transferring a frozen embryo in a later, unstimulated cycle sidesteps the risk entirely while preserving the same chance of a successful pregnancy.