If you’re considering undergoing IVF or egg freezing, you’re likely nervous about so many aspects of the process. The financial toll, the time commitment, the injections, the hormonal shifts, the emotional upheaval…and that’s not an even exhaustive list. But there’s one complication people may incur when undergoing fertility treatments, and it’s something not many people discuss.

The good news? Thanks to advances in the field of reproductive endocrinology, this complication in question is rare. We’re talking about ovarian hyperstimulation syndrome (OHSS), which, according to Mayo Clinic, is an exaggerated response to excess hormones.

“It is a syndrome that is most commonly caused by robust ovarian stimulation with hormones during the process of IVF or egg freezing (but there are also cases of a spontaneous occurrence),” says Jovana Lekovich, MD, a board-certified OBGYN/REI and medical director at RMA of NY. ”It is usually seen in patients who have very high ovarian reserve and have a lot of follicles growing.”

doctor treating a woman with ohss

Typically, symptoms of OHSS start about two days after an egg retrieval. It isn’t just IVF or egg-freezing patients who may be concerned about this possibility, though. According to Mayo Clinic, it can also happen in response to oral medications, such as Clomid, which is sometimes taken before attempting IVF or alongside IUI — though this possibility is far less common. 

Symptoms of mild-moderate OHSS may include abdominal pain, bloating, nausea, diarrhea, and tenderness near the ovaries. “In most severe cases (which we luckily don’t see today), it can lead to collection of fluid in the lungs, increased blood coagulability, and liver and kidney damage,” says Dr. Levovich. Even mild cases should be evaluated, so be sure to check in with your doctor if you experience any of these symptoms during fertility treatments. 

Dr. Lekovich explains how OHSS happens. “HCG (the pregnancy hormone) is one of the hormones that we use to induce maturity of the eggs prior to their retrieval. It is also a hormone that is produced during pregnancy,” she says. “In the older days of IVF, we used to see a lot more of these complications, as HCG was the only option we could use to induce maturity.”

woman smiling outdoors

But with advancements in reproductive medicine, fertility experts have changed protocols around IVF, which has helped reduce instances and severity of OHSS. Now, patients are able to trigger ovulation with drugs like Lupron, which can prevent or limit OHSS, for example. The rise of frozen embryo transfers during IVF is another major factor.

“Doing a fresh embryo transfer (which entails transferring embryos three or five days after the egg retrieval) was also the most common treatment option, as the science was not advanced enough for successful embryo freezing,” says Dr. Lekovich.

The expert adds that there are two types of OHSS: “Primary OHSS occurs when hCG is given as a maturation trigger, which can then lead to the release of another substance called VEGF (vascular endothelial growth factor) from the ovaries. VEGF leads to the symptoms of the condition,” she says. “Primary OHSS is a self-limiting condition and while uncomfortable, it is typically not dangerous for the patient.”

The more dangerous and severe version of the condition, secondary OHSS, typically occurs when someone undergoes a fresh embryo transfer and gets pregnant. The pregnancy hormone continues to rise in the patient’s blood, leading to an increase in VEGF production and worsening of symptoms. “We extremely rarely perform fresh transfers in the modern era of IVF, and really rely on frozen embryo transfers, so we luckily almost never see the secondary OHSS nowadays,” says Dr. Lekovich.

woman suffering from mild-moderate ohss

According to Mayo Clinic, OHSS can happen even in the absence of risk factors, but some factors that may increase a person’s likelihood of developing the condition include: Having PCOS, being under 35, having a large number of follicles, low body weight, previous instances of OHSS, and high or steeply rising levels of estrogen before an hCG trigger shot. 

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Back in the day, OHSS could have changed the trajectory of someone’s fertility journey during IVF. Now, with frozen embryo transfers being the typical course of treatment, that’s no longer the case. 

With that being said, Dr. Lekovich counsels patients about the risk of OHSS before they begin treatments. 

“Patients should understand that nowadays, OHSS is pretty rare (it occurs in 1% or less of the patients) thanks to using a Lupron trigger (which allows us to diminish the dose or completely omit the hCG trigger),” she says. “Additionally, it is important to understand that even if the OHSS happens, it will be the primary form related to the hCG trigger, which is self-limiting and not dangerous or life-threatening. They should look out for symptoms (which typically start 2 days after the egg retrieval, and sometimes worsen before they improve).” 

Zara Hanawalt is a freelance journalist and mom of twins. She's written for outlets like Parents, Marie Claire, Elle, Cosmopolitan, Motherly, and many others. In her (admittedly limited!) free time, she enjoys cooking, reading, trying new restaurants, and traveling with her family.