Ovarian Hyperstimulation Syndrome (OHSS): Symptoms, Risk, and When It's Serious

Ovarian hyperstimulation syndrome (OHSS) is an over-response to the fertility drugs used in IVF, in which the stimulated ovaries swell and fluid leaks out of the bloodstream into the abdomen, usually starting in the days after the trigger injection or egg collection. Most cases are mild and settle on their own. A small number become serious and need medical care. After my second egg collection I remember pressing on a bloated, tender stomach at 2am, convinced something was wrong, with no idea whether what I was feeling was normal or a warning. This is the plain explanation I wanted that night: what OHSS is, who gets it, and the exact signs that mean pick up the phone.

What OHSS actually is

OHSS happens because the drugs that stimulate your ovaries can, in some people, stimulate them too well. The medications used in an IVF cycle push the ovaries to grow many follicles at once instead of the single one a natural month produces. The trigger injection, which contains or mimics the pregnancy hormone hCG, then tips an over-responsive ovary into releasing chemicals that make blood vessels leaky¹. Fluid shifts out of the bloodstream and pools in the abdomen, which is what causes the bloating, the swollen ovaries, and, in serious cases, the more worrying symptoms. It is, in short, too much of the response the treatment is designed to create.

How common it is

Mild OHSS is common, affecting up to a third of IVF cycles, while the moderate and severe forms are much rarer. Most people who stimulate will feel at least some bloating and tenderness, and that mild version is expected rather than alarming. Moderate or severe OHSS occurs in roughly 1 to 5 in 100 cycles, and the truly severe cases that need hospital admission are rarer still². Clinics now watch for it so closely, with scans and blood tests through stimulation, that the serious end of the spectrum has become considerably less common than it once was.

Who is most at risk

The strongest risk factors are polycystic ovary syndrome, a high egg count, a high AMH level, and being younger. Ovaries that carry many small follicles to begin with, as in PCOS, tend to respond strongly to stimulation, which is why PCOS is the single biggest flag¹. A high anti-Mullerian hormone (AMH) result, a large number of follicles seen on scan, and more than about 15 to 20 eggs collected all point the same way, and younger patients (under about 35) are on average more responsive too. Knowing you carry one of these before you start lets your clinic plan a gentler protocol from day one.

The symptoms, mild to severe

OHSS runs on a spectrum, from bloating that settles in a week to symptoms that need urgent care. Mild OHSS brings bloating, mild tummy pain, and a swollen, tender abdomen in the days after egg collection³. Moderate OHSS adds nausea, vomiting, and more marked swelling. Severe OHSS is uncommon but is the reason this condition is taken seriously: rapid weight gain (sometimes a kilo or more in a day), severe or worsening abdominal pain, breathlessness, feeling faint, and passing much less urine than usual⁴. There is also timing to understand. Early OHSS tends to appear within about a week of the trigger. Late OHSS starts later and is driven by the hCG of an early pregnancy, so it can arrive just as you are hoping for good news, and it tends to be the more stubborn form.

How it is prevented and managed

The most effective prevention for higher-risk patients is to freeze all the embryos and transfer in a later cycle, so no pregnancy hormones stoke a fresh flare. Because late OHSS is worsened by the hCG of early pregnancy, delaying transfer until the ovaries have recovered removes that fuel; the frozen transfer that follows now carries success rates broadly comparable to a fresh one⁵. Alongside a freeze-all, clinics tailor the drug dose to your predicted response, monitor closely through stimulation, and for some patients use a different type of trigger injection or add medication to lower the risk. Mild cases are usually managed at home with rest, fluids, and simple pain relief, while severe cases are managed in hospital, sometimes by draining excess fluid and giving treatment to prevent blood clots.

When to seek urgent help

Rapid weight gain, severe bloating or abdominal pain, breathlessness, or passing much less urine are warning signs that need same-day contact, not a wait-and-see. Mild bloating that is slowly improving is expected and can be ridden out at home. What should never be sat on is a symptom that is escalating: a stomach that is getting tighter and more painful by the hour, shortness of breath, calf pain or swelling, or a clear drop in how often you are passing urine³. If you are unsure which side of that line you are on, that uncertainty is itself a reason to call, and clinics would far rather hear from you early. On that 2am night I sat on my worry until morning; I would tell anyone now to phone the clinic’s out-of-hours line instead, because that is exactly what it is for.

This is general information and support, not medical advice. Your own fertility team knows your protocol, your response, and your history, and is the right place to bring any symptom or worry about your own cycle.