Fertility medicines are used during treatments such as IVF to encourage the ovaries to produce several eggs. Occasionally, the ovaries respond more strongly than expected. This can lead to a condition called ovarian hyperstimulation syndrome, usually shortened to OHSS.

Most cases are mild and improve with monitoring and supportive care. However, moderate or severe OHSS can make someone very unwell and may require treatment in hospital. Updated 2026 guidance from the Royal College of Obstetricians and Gynaecologists emphasises early recognition, access to specialist advice and continued monitoring until symptoms have resolved. Read the RCOG guideline

What causes OHSS?

During fertility treatment, medicines stimulate the ovaries to develop eggs. A further hormone—often human chorionic gonadotrophin, or hCG—may be used to help the eggs mature.

In some people, the stimulated ovaries release substances that make small blood vessels more “leaky.” Fluid then moves out of the bloodstream and collects elsewhere, particularly in the abdomen. This can cause swelling, dehydration and thickening of the blood. In severe cases, it can affect the kidneys or breathing and increase the risk of a blood clot.

OHSS usually develops after an hCG trigger injection. It may appear:

• Within seven days of the injection: This is called early OHSS and is usually linked to a strong response to ovarian stimulation.
• Ten or more days later: This is called late OHSS and is often associated with an early pregnancy. Late OHSS may last longer and can be more severe.

How common is it?

The exact frequency is difficult to measure because mild and moderate cases are not always reported in the same way. The RCOG guideline advises patients that OHSS affects approximately 3–6% of people undergoing fertility treatment, although the risk varies according to the treatment used.

Severe and critical cases are much less common. In the UK, 66 cases were officially reported in 2021–22—fewer than 1 in every 1,000 IVF cycles.

Who is more likely to develop OHSS?

Anyone receiving ovarian-stimulation treatment can develop OHSS, but the risk may be higher in people who:

• Have polycystic ovary syndrome or polycystic-looking ovaries
• Have a high ovarian reserve, including a high AMH level or large number of follicles
• Are younger
• Have previously experienced OHSS
• Develop many follicles or have a high number of eggs collected
• Become pregnant during the treatment cycle
• Have a multiple pregnancy

The condition can occasionally occur after other forms of ovulation treatment. It is exceptionally rare in a naturally conceived pregnancy.

What symptoms should I look out for?

Possible symptoms include:

• Abdominal bloating or increasing abdominal size
• Abdominal discomfort or pain
• Nausea or vomiting
• Constipation or diarrhoea
• Rapid weight gain
• Passing less urine than usual
• Swelling of the legs or genital area
• Dizziness or fainting
• Shortness of breath

These symptoms are not unique to OHSS. Abdominal pain during fertility treatment could also be caused by an ectopic pregnancy, infection, appendicitis, a twisted ovary or a ruptured ovarian cyst. Severe pain or fever should therefore be medically assessed rather than assumed to be OHSS.

When should I seek urgent help?

Contact your fertility clinic or seek urgent medical advice if you develop:

• Increasing abdominal swelling or pain
• Persistent vomiting or difficulty keeping fluids down
• Noticeably reduced urine output
• Rapid weight gain
• Shortness of breath, particularly when lying flat
• Chest pain
• Fainting or severe dizziness
• Pain or swelling in one leg
• A severe headache, visual disturbance or unusual neck pain

If you attend an emergency department or another hospital, tell the healthcare team that you have recently received fertility treatment. Also inform your fertility clinic about any hospital attendance or admission.

Every fertility clinic should provide patients with written and verbal information about OHSS and a telephone number for advice that is available 24 hours a day.

How is OHSS diagnosed?

There is no single test that confirms OHSS. Diagnosis is based on your symptoms, examination and the results of appropriate investigations.

Assessment may include:

• Your weight and abdominal measurement
• Your pulse, blood pressure, temperature and oxygen level
• Blood tests to check blood concentration, kidney and liver function, salts and proteins
• Monitoring how much fluid you drink and how much urine you pass
• An ultrasound scan to assess the ovaries and look for fluid in the abdomen
• Additional chest or abdominal imaging if breathing problems or other complications are suspected

The healthcare team will also consider and exclude other conditions that could cause similar symptoms.

Can OHSS be managed at home?

Mild and moderate OHSS can usually be managed without admission to hospital. Carefully selected patients with severe OHSS may also receive outpatient care, provided they can attend regular reviews and have rapid access to hospital treatment if their condition worsens.

If you are being monitored at home, your clinical team may advise you to:

• Drink according to your thirst rather than forcing a fixed amount
• Keep a record of how much you drink and how much urine you pass
• Monitor your weight and abdominal swelling
• Remain gently mobile to reduce the risk of a blood clot
• Use prescribed pain relief and anti-sickness medication
• Attend follow-up appointments and blood tests

The guideline advises avoiding non-steroidal anti-inflammatory medicines, such as ibuprofen, because they may affect kidney function in people with OHSS. Ask your clinician which pain relief is safe for you.

Patients whose condition is stable may be reviewed every two or three days. Anyone whose symptoms worsen should receive an urgent clinical assessment. Most cases settle within approximately 7–10 days, although symptoms can last longer when pregnancy occurs.

When might hospital treatment be needed?

Hospital admission may be recommended if:

• Pain cannot be adequately controlled
• Vomiting prevents you from drinking enough
• Symptoms are worsening despite outpatient care
• You cannot attend regular follow-up appointments
• There are signs of complications
• Your individual or social circumstances make home monitoring unsafe

Hospital care may include intravenous fluids, pain relief, anti-sickness medication, regular blood tests and close monitoring of weight, abdominal size and urine output.

If a large amount of fluid has collected in the abdomen and is causing significant pain, breathing difficulty or reduced urine production, it may be drained using a procedure called paracentesis. This is performed under ultrasound guidance and can sometimes be undertaken without hospital admission.

Why are blood clots a concern?

OHSS can make the blood more concentrated and increase the likelihood of a blood clot. The risk is higher in moderate or severe OHSS, during pregnancy and in people with other risk factors such as reduced mobility, obesity or a known clotting condition.

Your team may recommend anti-embolism stockings and injections of a blood-thinning medicine called low-molecular-weight heparin. Everyone admitted with OHSS should normally receive preventative treatment unless there is a medical reason not to do so. The duration is tailored to the individual, particularly when pregnancy has occurred.

What happens to the embryo transfer?

If early OHSS symptoms develop before a planned fresh embryo transfer, the updated guideline recommends avoiding the fresh transfer. The embryos can instead be frozen and transferred during a later cycle, once the patient has recovered.

This “freeze-all” approach reduces the likelihood of late or worsening OHSS and does not appear to reduce the chance of a successful pregnancy.

Emotional support is important

OHSS can be physically uncomfortable and emotionally distressing. It may occur at an already demanding point in fertility treatment and can lead to uncertainty, delayed embryo transfer or hospital admission.

Patients should be offered clear information, regular contact and emotional support. Partners may also need reassurance and an opportunity to discuss their concerns.

The key message

OHSS is an uncommon but potentially serious complication of fertility treatment. Most patients recover fully with observation and supportive care, but symptoms can worsen quickly in some cases.

Know the warning signs, keep your fertility clinic’s emergency contact number available and seek advice promptly if you feel unwell. Early assessment and appropriate follow-up are the safest ways to prevent complications.

This article summarises the 2026 RCOG Green-top Guideline on the management of OHSS. It provides general information and should not replace personalised advice from your fertility or healthcare team.