What is OHSS?

OHSS is a complication of fertility treatment including in vitro fertilisation (IVF), where the ovaries become over-stimulated by hormone treatments given to mature eggs. It leads to leaky blood vessels and fluid building up in different parts of the body such as the abdomen and lungs. Although most OHSS is mild and get better with supportive treatment, more severe forms can potentially be dangerous and even life-threatening. 

What causes OHSS?

OHSS usually happens in women having in vitro fertilisation (IVF), a fertility treatment where eggs are retrieved from a woman and sperm from a man, which are then fertilised together outside of the body in a laboratory to form an embryo. The embryo is then transferred back to the woman’s uterus, to hopefully lead to pregnancy. During IVF treatment, hormones (usually follicle stimulating hormone; FSH) are used to stimulate multiple follicles (fluid-filled sacs that contain the eggs) in the ovary to grow. 

After the follicles have grown, eggs need be become mature so that they are able to be fertilised by sperm. This is done using an injection of the hormone human chorionic gonadotrophin (hCG), which is similar to the body’s luteinising hormone (LH) that normally peaks at ovulation. Human chorionic gonadotrophin (hCG) stays in the body for a long time after injection (more than one week), and sometimes it can over-stimulate the ovaries, leading to very large ovaries and release of chemicals (e.g. vascular endothelial growth factor; VEGF). This makes blood vessels in the body leaky and causes fluid from blood vessels to leak into areas of the body, such as the lungs and the abdomen.  

What are the signs and symptoms of OHSS?

OHSS can be either early (less than seven days following the hCG injection) or late (more than ten days after the injection). Late OHSS usually happens if the woman becomes pregnant following the fertility treatment, as hCG is also produced by the embryo and continues to stimulate the ovaries. Late OHSS can therefore take longer to get better, as the growing embryo continues to make hCG. 

OHSS can be mild or moderate, where symptoms are unpleasant but not life-threatening and usually go away by themselves. It can also be severe, where serious complications are more likely to develop and this may need admission to hospital. 

Symptoms of OHSS include feeling generally unwell, feeling sick (nausea), vomiting, abdominal (stomach) pain, abdominal swelling or bloating (because of fluid build-up in the abdomen and enlarged ovaries) and breathing difficulties (because of fluid build-up in the lungs).  

Women with OHSS also have a higher risk of developing dangerous blood clots in the legs (causing leg pain, redness or swelling), or lungs (causing chest pain, shortness of breath or palpitations), or brain (causing headaches, dizziness, or loss of vision). When the ovaries enlarge, they are at higher risk of twisting (called ‘ovarian torsion’), which can cause severe pain in the lower abdomen on either the right or left side.  

Signs of OHSS that doctors may see on blood tests and scans include abnormal kidney and liver function, fluid in the abdomen (ascites), fluid on the lungs (pleural effusion), enlarged ovaries, or blood clots in different arteries or veins in the body. 

How common is OHSS?

Up to one third of women having IVF develop a mild form of OHSS, and between 3-8% develop moderate or severe OHSS. Several factors increase a woman’s risk of developing OHSS: having polyendocrine metabolic ovarian syndrome (PMOS, previously called PCOS); being younger (under the age of 35yrs); having had OHSS in a previous pregnancy; and having lots of follicles in the ovaries or a high anti-Müllerian Hormone levels before starting IVF treatment (Hamoda et al 2026). 

Is OHSS inherited?

Although OHSS is almost always caused by hormones given during fertility treatment, rarely some women may be more likely to develop OHSS because of genes inherited from their parents. For example, some genes may cause a women’s ovaries to respond more strongly to the stimulating hormones (FSH and hCG), such as if the receptors for these hormones are more active. In addition, the likelihood of a woman developing PMOS (which is a risk factor for OHSS) is partly genetic. 

How is OHSS diagnosed?

There is no specific test for OHSS. It is diagnosed by taking a medical history (asking the woman about her current symptoms, and her fertility treatment), doing a physical examination (e.g. finding signs suggesting fluid in the abdomen and lungs), and doing an ultrasound scan of the pelvis (to look at the size of the ovaries) and abdomen (to look for fluid called ascites). Blood tests are also done to check the kidney function, protein levels, liver function and clotting. If there is concern about fluid in the lungs, an ultrasound scan of the lungs or X-ray of the chest can be done to look for this. If there is concern about a clot in the lungs, legs or brain, special scans such as a CT or Doppler ultrasound can be done of that specific body part. 

How is OHSS treated?

Some treatments such as dopamine agonists have been used to help OHSS, but the best approach is to avoid OHSS developing in the first place by tweaking the IVF treatment protocol. Most treatment for OHSS is supportive until it improves with time. Management of OHSS depends on the specific symptoms and complications that a woman has developed, and how severe they are. Most women with mild OHSS can be managed at home with supportive treatments such as anti-nausea medications, drinking plenty of fluids, and frequent visits to the IVF unit for monitoring until the symptoms get better. Depending on how severe the OHSS is, women may be given low dose blood-thinners to reduce the risk of developing a blood clot.  

If complications of OHSS develop, women may need admission to hospital, and specific treatment for that complication. For example, if there is excess fluid build-up in the lungs or abdomen, this can be drained using a tube placed directly into the lungs or abdomen. If kidney function is abnormal, women may need to receive fluids via a drip into the vein. If a blood clot in the lung or leg is found, women will need high dose blood-thinner medications. If a woman develops ovarian torsion (twisting of the ovary), she may need surgery to fix this. Rarely, women need to be admitted to the intensive care unit (ICU) if they are very unwell with OHSS. 

Are there any side-effects to the treatment?

The management for OHSS is treating specific symptoms or complications, and some treatments have specific side-effects or rare complications. For example, putting a tube into the lung or abdomen to drain excess fluid has a small risk of bleeding or infection, and therefore it is only done if the woman is very unwell. 

What are the longer-term implications of OHSS?

Usually, most women do not have any long-term effects after recovering from OHSS. Some studies have found an association between having OHSS and certain pregnancy complications such as premature birth; however, it is unclear if this is directly because of OHSS, or because women who develop OHSS are also more likely to have these pregnancy complications. Some women may have psychological trauma following OHSS, especially if they had a severe or life-threatening forms.  

If a woman has had OHSS in one pregnancy and goes on to have further IVF treatments, the doctors will likely change the IVF protocol to reduce the risk of over-stimulating the ovaries, by using lower doses and safer hormones such as GnRH agonists rather than hCG.