Ovarian hyperstimulation syndrome is a serious complication of ovarian stimulation and is characterised by enlarged ovaries, abdominal distention and discomfort, ascites, nausea and vomiting, and, in severe cases, oliguria, i.e. reduced urine output, liver dysfunction and respiratory distress syndrome.

The incidence of OHSS has been shown to increase with the number of oocytes retrieved. On the basis of the Society for Assisted Reproductive Technology (SART) data from 2008–2010, the incidence of OHSS was 0.37% in fresh cycles with six to 10 oocytes and 1.67% in fresh cycles with 16–20 oocytes.

The risk of OHSS must be balanced with the risks and costs of another cycle, however, most patients do not develop OHSS. Moreover, OHSS is becoming less of a concern owing to strategies developed to manage and prevent OHSS. For patients receiving a conventional GnRH antagonist protocol, the use of a GnRH agonist trigger seems to substantially reduce (but not eliminate) the risk of OHSS.