regimen are known to have or suspected of having a variety of health effects, some minor and some potentially major.

The most prominent side effect of ovarian stimulation is ovarian hyperstimulation syndrome. Its symptoms include increased ovarian size; nausea and vomiting; increased permeability of the blood vessels, leading to an accumulation of fluid in the abdomen; breathing difficulties; hemoconcentration, or an increased concentration of red blood cells; kidney and liver problems; and, in the most severe cases, blood clots or kidney failure. The severe cases affect only a very small percentage of women who undergo in vitro fertilization—about 0.1 to 0.2 percent of all treatment cycles—and the Class C severe, or the most dangerous, are an even smaller percentage. Only about 1.4 in 100,000 cycles leads to kidney failure, for example.

The OHSS risks for egg donors are expected to be much lower than the OHSS risks calculated from women involved in IVF. OHSS occurs at two stages: early, 3 to 7 days after the hCG trigger is used to prepare the eggs for retrieval, and is a result of that trigger; and late, 12 to 17 days after the trigger, and is a result of the new pregnancy in a women who has successfully undergone IVF. The risk of severe complications is about 4 to 12 times higher in late-onset OHSS than in early-onset OHSS. Egg donors, because they will not be getting pregnant after donating their eggs, will not be affected by the late-onset OHSS and thus can be expected to have many fewer side effects than are seen in IVF patients.

Many observers have worried that the use of fertility drugs could lead to an increased risk of cancer—in particular, breast, ovarian, and uterine (including endometrial) cancers. One must be careful in interpreting epidemiological studies of women taking fertility drugs, because all of these cancers are more common in women with infertility, so merely comparing women taking fertility drugs with women in the general population inevitably shows an increased cancer risk. When the analysis is done correctly, accounting for the increased cancer risk due to infertility, the evidence does not support a relationship between fertility drugs and an increased prevalence of breast or ovarian cancer. More research is required to examine what the long-term impact fertility drugs may be on breast and ovarian cancer prevalence rates. For uterine cancer, the numbers are too small to achieve statistical significance, but it is at least possible that fertility drugs may indeed cause some increased risk of uterine cancer.



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