Short answer
Ovarian hyperstimulation syndrome (OHSS) is an over-response to the hormone medicines used to stimulate the ovaries in fertility treatment, most often IVF. The ovaries become enlarged and fluid can leak into the abdomen, causing swelling, pain and nausea. Mild OHSS is common and usually settles on its own, while moderate or severe OHSS affects just over 1 in 100 people having IVF and can, rarely, be life-threatening. How likely it is depends mainly on how strongly the ovaries respond and whether a pregnancy follows.
Key points
- Up to a third of people having IVF may get mild OHSS.
- Moderate or severe OHSS affects just over 1 in 100.
- Strong symptoms usually appear in the week after egg collection.
- Polycystic ovaries, age under 30 and previous OHSS raise the risk; pregnancy can make it worse.
- Clinics can lower the risk through the choice of stimulation protocol and trigger injection.
Why OHSS happens
In IVF, gonadotrophins (hormone injections that stimulate egg production) are given so that several eggs mature in one cycle. In people who are very sensitive to these medicines, too many follicles develop and the ovaries become large and painful.1
In more significant OHSS, fluid shifts out of blood vessels into the abdomen and sometimes around the lungs. That explains the swelling, the thirst and dehydration, and, in severe cases, the breathlessness and reduced urine output.2
How common is it, and how severe can it get?
The Royal College of Obstetricians and Gynaecologists (RCOG) estimates that as many as 33 in 100 people having IVF get mild OHSS, and just over 1 in 100 develop moderate or severe OHSS.2 The HFEA, the UK fertility regulator, notes that severe OHSS is very rare but can be life-threatening.1
| Severity | Typical symptoms | Usual care |
|---|---|---|
| Mild | Abdominal swelling, discomfort, nausea2 | Usually managed at home with pain relief1 |
| Moderate | More swelling from fluid in the abdomen, pain, vomiting2 | Closer monitoring; anti-sickness medicine and fluids2 |
| Severe | Extreme thirst, dark or reduced urine, breathlessness from fluid around the lungs; rarely blood clots2 | Often hospital care, intravenous fluids, possible drainage, clot prevention2 |
Who is more at risk?
The RCOG lists polycystic ovaries, being under 30 and having had OHSS before as risk factors.2 Becoming pregnant in the same cycle also matters, especially with twins or more, because pregnancy hormones can make OHSS worse and longer lasting.2
Without a pregnancy, OHSS usually improves by the next period, and most symptoms settle within 7 to 10 days. If pregnancy occurs, it can persist for a few weeks or longer.2 People who develop strong symptoms usually do so in the week after egg collection.1
Can OHSS be prevented?
It cannot always be prevented, but the risk can be reduced. Two approaches have been tested in Cochrane systematic reviews, which pool the results of randomised trials.
The first is the stimulation protocol. A 2016 review of 73 trials found that a GnRH antagonist protocol (a short course of medicine that blocks premature ovulation) led to less OHSS than a long GnRH agonist protocol. If the agonist risk was 11%, the antagonist risk would be about 6% to 9%, with no evidence of a difference in live birth rates.4 The review rated this evidence as moderate quality, so the evidence for the reduction is Moderate.
The second is the trigger injection that finally matures the eggs. A 2014 review found that using a GnRH agonist trigger instead of hCG (human chorionic gonadotrophin) markedly reduced OHSS. However, live birth rates were lower when embryos were transferred fresh in the same cycle, so the authors suggested this approach may suit people who choose to avoid a fresh transfer and freeze embryos instead.5 This trade-off is one reason clinics sometimes advise freezing all embryos when the ovaries have responded very strongly.
Clinics should also give written information about OHSS and a 24-hour contact number before treatment starts.2
How OHSS is treated
There is no treatment that switches OHSS off; care relieves symptoms and prevents complications while it resolves. Options include pain relief, anti-sickness medicine and intravenous fluids.2
Because OHSS raises the risk of blood clots, support stockings and heparin injections (a blood thinner) may be advised. If fluid builds up, paracentesis (draining fluid from the abdomen with a thin needle guided by ultrasound) can ease discomfort and help breathing and kidney function. Hospital admission may be needed when pain or vomiting cannot be controlled or the condition is not improving.2
When to seek medical advice
Contact your fertility clinic straight away if you have a swollen or painful stomach, nausea or vomiting, shortness of breath, faintness or are passing less urine than usual.1 If you cannot reach the clinic, contact your GP, NHS 111 or A&E.2
Call 999 or go to A&E, and do not drive yourself, if you have difficulty breathing, chest or upper back pain, swelling in any part of your body, or severe pain low down on one side of your tummy, which can also be a sign of ectopic pregnancy.3
Common questions
Does OHSS mean the IVF cycle is cancelled?
Sometimes; the NHS notes that the current cycle may need to stop.3 Some clinics instead freeze the embryos and transfer one in a later cycle, which our page on frozen versus fresh embryo transfer explains.
Can egg freezing cause OHSS?
Yes, because egg freezing uses the same ovarian stimulation as IVF.1 See what to know about egg freezing.
Am I wasting the clinic’s time if symptoms are mild?
No; the HFEA specifically says you should never feel you are wasting the clinic’s time by calling.1
How long does heparin need to continue?
The RCOG leaflet says it is usually continued for 7 days after symptoms resolve, or until the end of the 12th week if you are pregnant; your clinician will confirm the plan.2
Bottom line
OHSS is a recognised side effect of ovarian stimulation: often mild and short-lived, occasionally serious. Knowing the warning signs, and asking how a clinic plans to reduce the risk if you are likely to respond strongly, are the most useful steps.
Related reading
- IVF guide
- Frozen vs fresh embryo transfer: is one better?
- What should you know about egg freezing?
- What does an AMH test tell you?
- Why do clinics recommend single embryo transfer?
Sources
- Human Fertilisation and Embryology Authority. Risks of fertility treatment. HFEA. https://www.hfea.gov.uk/treatments/explore-all-treatments/risks-of-fertility-treatment/. Accessed 5 November 2026.
- Royal College of Obstetricians and Gynaecologists. Ovarian hyperstimulation syndrome: patient information leaflet. RCOG; 2016. https://www.rcog.org.uk/for-the-public/browse-our-patient-information/ovarian-hyperstimulation-syndrome-patient-information-leaflet/. Accessed 5 November 2026.
- NHS. IVF: risks. NHS website. https://www.nhs.uk/conditions/ivf/risks/. Accessed 5 November 2026.
- Al-Inany HG, Youssef MA, Ayeleke RO, Brown J, Lam WS, Broekmans FJ. Gonadotrophin-releasing hormone antagonists for assisted reproductive technology. Cochrane Database of Systematic Reviews; 2016, Issue 4: CD001750. https://www.cochrane.org/evidence/CD001750_gonadotrophin-releasing-hormone-antagonists-versus-gnrh-agonist-subfertile-couples-undergoing. Accessed 5 November 2026.
- Youssef MAFM, et al. Gonadotropin-releasing hormone agonist versus HCG for oocyte triggering in antagonist-assisted reproductive technology. Cochrane Database of Systematic Reviews; 2014: CD008046. https://www.cochrane.org/evidence/CD008046_gonadotropin-releasing-hormone-agonist-versus-hcg-oocyte-triggering-antagonist-assisted-reproductive. Accessed 5 November 2026.
Article history
- — First published.