In the stimulated cycle hormonal injections are used to force all available follicles, in both ovaries, to grow in a synchronous manner, so that we can retrieve more than one oocyte at egg collection (optimal number: 5-15). In the natural cycle, no hormones are administered, and the dominant follicle of that cycle grows with the woman’s itself hormonal secretion, as these are naturally secreted from the pituitary gland in the brain. As a result, we can only obtain one oocyte at egg collection. We must notice that in 30% approximately of natural cycles, at al women ages, no egg is found when we insert the collection needle inside the dominant follicle.

During the natural cycle the circulating estrogen in blood (estradiol) does not exceed “physiological” levels (100-350 units) and no chance of ovarian hyperstimulation exists. This is preferable in women with a history of gynecological malignancy (e.g. Breast, cervical, ovarian) where any supra-physiological increase in blood estrogens may be harmful. A possible disadvantage is that in 30% of cases no oocyte is obtained, or it fails to fertilize with the sperm, so there is no embryo to transfer in the uterus. This leads to repeated egg collections to have one or two embryos for transfer, which increases the cost and the emotional stress for the couple. If ovarian hyperstimulation is not contra-indicated, the harvest of more than one oocyte (preferred number: 5-14) after a stimulated cycle, leads to the culture of two or more embryos, where the embryologist (using the technically advanced method of embryoscope) can choose the best quality embryos (either day 3 or day 5-blastocyst) to transfer in the uterine cavity. Thus, the chances of pregnancy are maximized in one, stimulated, cycle. As a disadvantage remains the possibility of hyperstimulation syndrome, especially in women with polycystic ovaries. Close monitoring of the follicular growth (by ultrasound and hormonal blood test) diminishes the risk of hyperstimulation.