Ovarian hyperstimulation syndrome (OHS) is a condition where a woman receiving fertility drugs (usually gonadotropins) over-responds by developing a large number of ovarian follicles which upon administration of hCG, “triggers” a series of systemic events that can place the woman’s health and even her very life at risk. The development of OHS is linked to the effect of hCG and thus does not arise until the “hCG trigger” is administered. In fact, the woman is not at risk until the hCG is administered.. If a woman who develops OHS does not conceive then within 10-14 days of hCG administration, as the hormone clears the woman’s body, the condition spontaneously and very rapidly resolves its own. If on the other hand she conceives, then the severity of OHS can worsen and can progress to the point that a large amount of fluid collects in her abdominal cavity (ascites), her kidneys, lungs, and heart are compromised, to the point of her life being placed at risk (severe ovarian hyperstimulation syndrome –OHSS). This risk increases progressively as the hCG production by the developing root system (placenta) of the conceptus “fans the flames”. The good news however, is that even if she does conceive and develops OHSS, with its serious risks, the condition will nevertheless self-resolve 7-8 weeks into the pregnancy. The challenge therefore, is to institute measures that will keep overstimulated women out of harm’s way by delaying the “hCG trigger” until they are out of risk. To do so, requires a women with 25 or more ovarian follicles be critically reassessed for OHSS risk-factors prior to  receiving the “hCG trigger” (see “preventing OHSS” …elsewhere)  and that women who have undergone egg retrieval and thereupon are deemed to be at risk of developing OHSS be critically reassessed for that risk, prior to undergoing a fresh embryo transfer. In fact, in the event of the latter it is advisable to wait until 5-6 days post ER (when viable embryos will have reached the blastocyst stage) in order to allow for the maximum time possible before making the final decision as to whether to proceed to a fresh embryo transfer versus deferring the embryo transfer to a subsequent cycle (FET). Mild OHS: A large percentage of women undergoing controlled ovarian stimulation (COS) with administration of the hCG trigger, will subsequently experience mild abdominal distention with a modest collection of fluid in the pelvis. This can be accompanied by some discomfort due to bloating and ovarian enlargement. If no pregnancy occurs, all such symptoms and signs dissipate within 12-14 days of the hCG trigger. If pregnancy ensues, it sometimes worsens and thereupon dissipates as ovarian function is taken over by the developing placenta, around the 8th-9th week of pregnancy. Mild OHS is never an indication to cancel and defer embryo transfer. Moderately severe OHS: Here, the large number of emptied (aspirated) ovarian follicles in the hyperstimulated ovaries re-fill with fluid and become significantly enlarged. In such cases, there will usually be a moderate amount of free fluid in the abdominal cavity (ascites) even in the chest cavity (pleural effusion). There will usually not be vomiting, diarrhea, diminished urine flow severe abdominal pain and shortness of breath in cases of moderate OHS. Here again, if following embryo transfer no pregnancy occurs, symptoms will get worse for about 7 days, plateau in the ensuing 4 days and then dissipate 12-14 days of the hCG trigger. Should pregnancy ensue, it will worsens but will resolve as ovarian function is taken over by the developing placenta, around the 8th-9th week of pregnancy. In many such cases physicians will elect to proceed with fresh embryo transfers. However, with recent advances in embryo cryopreservation (vitrification) where frozen embryo transfers (FET) now yield results that are at least as good as with the transfer of “fresh” (unfrozen) embryos and the fact that using vitrification, 80-90% of frozen embryos will survive the freeze-thaw in the same condition as the time they were frozen, there is a strong movement towards freezing embryos and the performing FET in a subsequent cycle once the patient is completely out of risk. Severe ovarian hyperstimulation syndrome (OHSS): OHSS presents with these symptoms too, but these are usually much more severe with the abdomen becoming much distended with fluid (clinical ascites), often to the point of causing severe pain and shortness of breath. The latter is due to the combined effects of gross ovarian enlargement and ascites that “splints” the diaphragm and cause labored breathing. The increased intra-abdominal pressure also commonly exerts pressure on the upper gastrointestinal tract causing the stomach to slide through the diaphragmatic opening through which the esophagus passes, leading to a functional hiatal hernia with gastric reflux, pain and often also vomiting. In addition, ovarian enlargement stimulates the vagus nerve. This can cause a slowing of the heart rate (bradycardia), sweating, diarrhea, and vomiting. These effects alone are sufficient to warrant that the woman with OHSS undergo, thorough hematologic, biochemical and physical evaluation, and sometimes that she be admitted to hospital for treatment and close observation. It is also advisable in all such cases, that all embryos be cryostored so that the embryo transfer can be done in a subsequent hormone-prepared cycle after the OHSS has resolved. Transvaginal draining excessive abdominal fluid (paracentesis) : In cases of severe, symptomatic ascites that causes significant pain, and difficulty in breathing, paracentesis affords immediate symptomatic relief and, since it relieves the buildup of intra-abdominal pressure, reducing the compression of blood vessels it allows for improved liver, kidney and intestinal function. The effect is transient and only lasts a few days. Thus paracentesis might have to be repeated every few days (as needed) until the condition resolves.