OHSS, or ovarian hyperstimulation syndrome, is an excessive ovarian response to the medication used in ovarian stimulation, in which follicles grow beyond what was intended and oestradiol levels rise sharply. That hormonal excess makes blood vessels more permeable, so fluid leaks into the abdominal cavity and causes swelling, discomfort and disturbances in fluid and salt balance. Most cases are mild and resolve on their own. Severe forms are uncommon, and modern protocols prevent them well.
What causes OHSS
The trigger is the medication that induces final follicular maturation, particularly when human chorionic gonadotropin (hCG) is used in an ovary that has already responded strongly. That hormone keeps the stimulus active for days, and that is where fluid accumulates.
The distinction that matters is between the drug trigger and the body’s own hCG. If an embryo implants, the pregnancy produces its own hCG, and the stimulus does not fade: it grows. That is why the timing of a transfer is part of preventing OHSS, not a separate decision.
Who is at higher risk
Risk is estimated before stimulation begins, and it is what determines the starting dose. An ovary predicted to respond strongly is stimulated with less medication from day one; it is not corrected along the way.
Higher risk applies to:
- High ovarian reserve. More follicles are available from the outset. The markers that anticipate this are covered in the AMH test entry and in the guide on how many follicles are normal.
- Polycystic ovary syndrome, for the same reason.
- Younger and slimmer women, whose ovaries tend to respond more intensely.
- Anyone who has had OHSS in a previous cycle.
OHSS symptoms: mild, moderate and severe
This is where the most common confusion sits. A normal stimulation cycle already causes discomfort: abdominal bloating, heaviness, breast tenderness and some fluid retention in the final days. The ovaries have grown, and that is felt.
The syndrome begins when those symptoms, instead of settling after egg retrieval, get worse. The practical difference is not the symptom itself but the direction: what is expected improves, the syndrome worsens.
The vast majority of cases are mild. Severe forms affect a small proportion of cycles, and that proportion keeps falling with current protocols.
- Mild. Bloating, abdominal discomfort and some weight gain. Managed at home with hydration, a lower-salt diet, relative rest and telephone follow-up. It resolves on its own.
- Moderate. The above plus nausea, vomiting and fluid in the abdomen visible on an ultrasound scan. It needs closer clinical monitoring and sometimes serial blood tests.
- Severe. Uncommon. There is abundant abdominal fluid, a marked drop in urine output, abnormal blood results and a risk of vascular complications. It requires hospital admission.
When to seek urgent medical help
You do not need to be certain of anything before making the call. Contact your clinic the same day if any of these appear. They are observable signs, and they do not depend on interpreting a pain:
- Your abdomen grows from one day to the next and your clothes stop fitting.
- You gain two kilos or more in two or three days.
- You are passing much less urine than usual, and it is dark.
- It is hard to breathe when lying down, or you need to sleep propped up.
- Repeated vomiting that stops you drinking.
- Abdominal pain that keeps increasing instead of easing.
The first call is to the clinic running your cycle, which can arrange a same-day ultrasound scan. Go straight to A&E if you are breathless at rest, if the pain is sudden and severe, or if you cannot reach anyone.
What helps at home when it is mild
The mild form is managed outside hospital, and the practical questions repeat themselves:
- Drink to thirst, without forcing large volumes of plain water. Drinks containing salts help fluid stay where it belongs, which is why isotonic drinks are suggested. This is supportive care, not a cure.
- Meals with protein and less salt. Salt encourages retention.
- Weigh yourself each morning, under the same conditions. It is the most reliable signal and the easiest to follow: what matters is not the weight but how much it rises and over how many days.
- Relative rest, not bed rest. Gentle walking helps circulation; complete rest is not indicated unless prescribed.
- Avoid high-impact exercise and sudden movements. With enlarged ovaries there is a risk of torsion.
- No anti-inflammatories on your own initiative. They can interfere with kidney function exactly when it matters most, so check any painkiller first.
None of these measures cures the condition. They support you while it resolves and make it easier to spot in time if it stops improving.
How long does OHSS last: early and late
Duration depends on which of the two forms appears, and this distinction is rarely explained:
- Early OHSS. Appears three to seven days after egg retrieval and is triggered by the maturation medication. It usually resolves in one to two weeks, once your period arrives and hormone levels normalise.
- Late OHSS. Appears beyond the first week and is triggered by the hCG of the pregnancy itself, if implantation has occurred. It is less common but tends to last longer and be more intense, because the hormonal stimulus does not disappear: it increases.
The reference event is therefore not the same. Early OHSS is counted from egg retrieval, late OHSS from implantation.
How OHSS is prevented
This is the part that most patient information leaves out, and it is the one that changes what happens to you. Prevention is the design of the cycle, not a late reaction. The available levers:
- Setting the dose from the start according to the estimated ovarian reserve. It is the most effective measure and the earliest one.
- An antagonist protocol, which leaves the option of changing the final maturation medication if the response escalates.
- Triggering with a GnRH agonist instead of hCG when risk is present. It substantially reduces the likelihood of the severe form.
- Cabergoline, a drug that reduces vascular permeability and with it the leakage of fluid.
- Coasting, which means withholding gonadotropins for a few days while maintaining suppression, so levels fall before final maturation.
- Freezing all embryos instead of transferring fresh.
That last one deserves a moment. When there is a risk of hyperstimulation, embryos are not transferred fresh: they are all vitrified, your period is allowed to arrive and hormone levels to settle, and the transfer happens in a later cycle. It avoids the worst scenario, which is adding the hCG of a pregnancy to an already overstimulated ovary, and it does not penalise results. It is the reason a transfer that seemed ready is sometimes postponed.
The Royal College of Obstetricians and Gynaecologists publishes both patient information and a clinical guideline on the management of OHSS, and its criteria are the reference used in the UK.
Treatment if you develop OHSS
Mild cases need supportive care and monitoring, as described above. Moderate cases add closer follow-up, serial blood tests and ultrasound scans to track the fluid.
Severe cases are treated in hospital. Management centres on restoring fluid balance intravenously, monitoring kidney function and blood concentration, preventing thrombosis and, where the fluid is causing breathlessness or significant pain, draining it. It resolves, but it needs to be managed where those things can be done.
OHSS and pregnancy
A pregnancy can progress perfectly normally. What does happen is that the hCG of the pregnancy can prolong or intensify the condition, which is precisely why a later transfer is preferred when risk is high.
That is also why late OHSS and a positive pregnancy test often arrive together, and why worsening symptoms after a positive test should be reported rather than put down to early pregnancy.
Frequently asked questions about OHSS
How long does OHSS last?
Early OHSS usually resolves in one to two weeks, once your period arrives. Late OHSS, linked to the hCG of a pregnancy, lasts longer because the hormonal stimulus continues as the pregnancy advances.
Can OHSS be prevented?
Not entirely, but the risk falls substantially by setting the dose from the start, choosing an agonist trigger, using cabergoline where appropriate and freezing embryos instead of transferring fresh.
Does OHSS affect the pregnancy?
The pregnancy can progress normally. What changes is that the hCG of the pregnancy can prolong or intensify the condition, which is why a later transfer is preferred when the risk is high.
Why has my transfer been cancelled because of OHSS?
Because transferring fresh would add the hCG of a pregnancy to a stimulus that is already there. Freezing the embryos and transferring them later avoids that combination without giving up a single embryo.
Does OHSS always appear after egg retrieval?
No. The early form appears in the first days after retrieval, but many cycles involve only the normal discomfort of stimulation, which settles on its own.
Can OHSS be seen on a scan?
Yes. An ultrasound scan shows the size of the ovaries and the presence of fluid in the abdomen, which is what allows the severity to be graded and the management decided.
Is OHSS more likely with PCOS?
Yes. Polycystic ovaries hold a larger pool of small follicles, so they tend to respond more intensely to stimulation. It is one of the reasons the starting dose is lowered when PCOS is present.
