What Is the Ovarian Hyperstimulation Syndrome (OHSS)?

By BSc, MSc (embryologist) and BA, MA (fertility counselor).
Last Update: 04/23/2014

The ovarian hyperstimulation syndrome is an abnormally exaggerated ovarian response to the administration of hCG (human chorionic gonadotropin) after a cycle of controlled ovarian stimulation.

Below you have an index with the 4 points we are going to deal with in this article.


An increase in the permeability of blood vessels causes an ovarian enlargement and the acute extravasation of fluids from bloodstream. The function of the hepatic, hematologic, kidney, and respiratory systems is compromised; therefore, severe OHSS can be considered as life-threatening.

Its frequency varies from 0.6 up to 14% among women undergoing ovulation induction treatments. Severe OHSS incidence ranges from 0.2-1.8% and that of moderate OHSS is 3.1-6%. Furthermore, it is more frequent in cycles where the woman becomes pregnant.

Types of OHSS

Depending on the time it takes to appear, it can be classified in two different forms:

Early OHSS
3-7 days since the administration of hCG.
12-17 days since the administration of hCG. It usually occurs when pregnancy occurs.


It is usually caused after the administration of gonadotropins or if endogenous LH levels increase naturally.

This syndrome is very limited in duration, since it disappears after the beginning of menstruation. However, it should be under control and an appropriate medical follow-up should be done through transvaginal ultrasounds, to be sure more serious complications won't appear.

Risk factors

Risk depends on each individual case, mainly because it may be affected by factors such as age, each person's muscle mass, if you have ever been pregnant, if it is a case of multiple births, etc. Should you need further information about guessing your own situation and which risk factors most affect you, please have a look at this section.

Like in any other illness or health problem, it is rational to follow a 'better safe than sorry' strategy. Thus, we give you some tips here so that both doctor and patient are able to prevent, as far as possible, the development of ovarian hyperstimulation syndrome (OHSS).

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 Neus Ferrando Gilabert
Neus Ferrando Gilabert
BSc, MSc
Bachelor's Degree in Biology from the University of Valencia (UV). Postgraduate Course in Biotechnology of Human Assisted Reproduction from the Miguel Hernández University of Elche (UMH). Experience managing Embryology and Andrology Labs at Centro Médico Manzanera (Logroño, Spain). More information about Neus Ferrando Gilabert
Adapted into english by:
 Sandra Fernández
Sandra Fernández
Fertility Counselor
Bachelor of Arts in Translation and Interpreting (English, Spanish, Catalan, German) from the University of Valencia (UV) and Heriot-Watt University, Riccarton Campus (Edinburgh, UK). Postgraduate Course in Legal Translation from the University of Valencia. Specialist in Medical Translation, with several years of experience in the field of Assisted Reproduction. More information about Sandra Fernández

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