(OHSS) A complication of ovulation induction with, usually, follicle stimulating hormone, especially in cycles of superovulation for assisted conception, when it is intended to retrieve more than one egg. The ovaries become large, they can be painful, and there is excessive fluid released into the abdomen (the peritoneal cavity). Either removing this fluid or the occurrence of vomiting can cause dehydration, thickening of the blood and, occasionally, a serious thrombosis, such as a stroke. Death has been reported. Moderate to severe OHSS is treated in hospital, with administration of fluid intravenously, sometimes including albumin.

(hCG) A gonadotropin produced by the placenta in pregnancy (specifically it's produced by the trophoblast of the chorionic villi); the hormone measured in performing a pregnancy test. The generic (no frills') name for Pregnyl and Profasi, which are preparations of hCG obtained by extracting it from the urine of pregnant women, and Ovidrel, which is made by recombinant gene technology. Mimics the action of luteinising hormone (LH), but has a very much longer duration of action -- and this gives hCG considerable advantages over LH in clinical use. Given as an injection to lead to ovulation from a mature follicle 38 hours after the injection; or to stimulate ongoing function of the corpus luteum, particularly its production of progesterone. So it is typically given after a course of follicle stimulating hormone (FSH) in assisted conception (IVF or GIFT) programs and ovulation induction programs 36 hours before the expected time of egg retrieval (or before having sex or IUI), and then sometimes in further, smaller doses to support the luteal phase that follows. Sometimes used with clomiphene. Ovarian monitoring is needed for its correct use with FSH or with clomiphene. hCG treatment can precipitate the ovarian hyperstimulation syndrome (OHSS).

Early experience with superovulation for in vitro fertilisation, particularly at the original US program in Norfolk VA, saw women patients classified into three classes of responders, depending on how many eggs were obtained at egg retrieval. Poorer than usual outcomes could occur with either a low response or a high response (the high responders generally have polycystic ovaries, with a risk of ovarian hyperstimulation syndrome (OHSS). Many maneuvers have been used to try and retrieve more eggs among low responders, such as starting high dose FSH on day 1, the adding of growth hormone, the avoidance of GnRH-agonists etc., all generally without improving the results. Day 3 serum FSH levels can be high in low responders, indicating a depletion of eggs in the ovaries.

Twisting of an organ on its vascular pedicle, risking necrosis. Can happen to the testis in boys, especially after an injury to the scrotum, or to the ovary and fallopian tube, either spontaneously during adolescence or when enlargement from superovulation has produced ovarian hyperstimulation syndrome (OHSS). An operation to untwist it can save the affected organ, if carried out quickly enough.

(VEGF) A growth factor (or local hormone) needed for new blood vessels to form in, for example, the corpus luteum, as it turns into an efficient hormone-producing gland making and releasing progesterone about a week after ovulation. Because such new blood vessels are rather leaky, excess VEGF from multiple corpora lutea during ovulation induction or superovulation using gonadotropins can be associated with new blood vessels trying to form in the general peritoneal cavity, leakage of fluid from which causes fluid to build up in the abdomen, a potentially dangerous condition known as the ovarian hyperstimulation syndrome. VEGF is also important in the formation of the placenta, so mutations of VEGF or its target proteins can cause recurrent miscarriage.