Ovarian Hyperstimulation Syndrome (OHSS) is “the” IVF complication. It is almost entirely unique to fertility treatment, although a few cases have been cited where OHSS has occurred in a naturally occurring pregnancy.

What is OHSS? How is it classified?

Ovarian stimulation… Isn’t that the point? Those of us familiar with the IVF process will recognise ovarian stimulation as one of the steps of IVF. This is when medications are given to encourage follicles in the ovaries to mature, grow and produce oocytes (eggs) prior to egg collection. On the other hand, OHSS is what happens when the ovaries react too much, to the point where it causes wider problems across the body.

Like many other conditions, OHSS varies in severity, and severity is one of the ways in which we classify OHSS; mild, moderate, severe, and critical.

Ovarian Hyper-stimulation Syndrome (OHSS) Classification Summary

Generally, as the size of the ovaries increase, the symptoms worsen, and thus the severity of OHSS increases. Increased severity also conveys an increased complication risk.

Mild OHSS is quite common, occurring in around 1 in 3 IVF cycles, and the symptoms are bloating and mild abdominal pain. Mild cases of early OHSS are often difficult to distinguish from the side effects of egg collection.

Mild Ovarian Hyper-stimulation Syndrome (OHSS)

With moderate OHSS there is usually more pain, and patients tend to feel and/or be sick. Fluid build-up in the abdomen (ascites), as seen on an ultrasound scan, is also associated.

Moderate Ovarian Hyper-stimulation Syndrome (OHSS)

As we reach severe OHSS, the fluid build-up becomes visible to the naked eye. There may also be fluid in the chest as well. At this point there are also derangements in blood tests due to the fluid shifting away from the blood and into the abdomen and chest.

Severe Ovarian Hyper-stimulation Syndrome (OHSS)

Critical OHSS is rare but is associated with life-threatening complications such as kidney failure, acute respiratory distress syndrome (where the fluid build-up in the lungs affects the levels of circulating oxygen), blood clots and even ovarian rupture and subsequent haemorrhage.

Critical Ovarian Hyper-stimulation Syndrome (OHSS)

Early or Late?

Another way in which OHSS is classified is according to the time of onset; Early or Late.

Early OHSS occurs during treatment, usually within 7 days of the trigger (HCG) injection for egg collection. Early OHSS is thought to be due to the ovaries themselves reacting too much.

Late OHSS occurs 10 or more days after the trigger injection, usually in association with an early pregnancy. This makes sense because HCG is the pregnancy hormone that gives us positive pregnancy tests, and because it is also what we use to give the ovaries the final push prior to egg collection, these are the two points in time at which OHSS occurs.

What are the risk factors?

As OHSS starts at the level of the ovaries, so do the risk factors. Risk factors for OHSS include:

  • Polycystic Ovary Syndrome (PCOS)
  • Previous OHSS
  • A high number of follicles before treatment (antral follicle count or AFC)
  • High ovarian reserve blood marker (anti-Müllerian hormone or AMH).

Patients at high risk for OHSS can have their treatment altered accordingly – they can be offered different medications or put on a ‘short protocol’ (also known as an antagonist protocol). Short protocols help to reduce the risk of OHSS by minimising the amount of time the stimulating medications are used, effectively giving these already more active ovaries less time to over-stimulate.

What can be done about it?

OHSS is, by and large, a self-limiting condition. This means that it resolves with time and without the need for any specific treatment.

The management of OHSS depends on the severity. Patients with mild OHSS are usually monitored by their fertility clinic on an outpatient basis.

A large focus of our treatment is on keeping patients comfortable – in other words, managing the symptoms. Patients with more severe cases of OHSS (usually severe and critical cases, but decisions are made by fertility clinics on a case-by-case basis) are usually admitted for monitoring and relevant treatment as necessary. This may include rehydration via intravenous fluids, draining the excess fluid from the abdomen or chest, or the use of a blood thinner if they are at an increased risk of clotting

How does this impact my treatment cycle or my pregnancy?

The ways in which OHSS impacts the fertility treatment process depends on severity and timing. In Early OHSS (pre-pregnancy) – cases where the patient becomes more unwell may require freezing all the embryos collected and using them for a subsequent frozen cycle.

Late OHSS has been shown to increase the risk of the pregnancy complications pre-eclampsia (a disorder that develops after 20 weeks of gestation and is characterised by high blood pressure that affects multiple organ systems – most commonly the kidneys, resulting in protein in the urine) and preterm delivery. OHSS has not been shown to increase the risk of miscarriage.

Take home message

OHSS is a complication of fertility treatment that can have a varying level of impact on treatment cycles, ranging from causing discomfort to paused treatment cycles (freezing all embryos), and can even be life-threatening in rare cases. There are several known risk factors that are routinely tested for, and treatment regimens can be adjusted accordingly to minimise the risk.



Fourth-year medical student at Newcastle University passionate about patient education and empowerment. Interests include Obstetrics & Gynaecology, Reproductive Medicine, and Sexual Health.

Matt is an NHS Consultant in Newcastle with over ten years of experience. His PhD research into subfertility and miscarriage involved developing a clinical trial and patient engagement.