In a regular ovulation cycle, a hormone signal (called Follicle Stimulating Hormone (FSH)) from the pituitary is sent out at the beginning of the cycle (during menstruation) and it leads to the random selection of a follicle, which enlarges over the course of 1-2 weeks and eventually releases as a mature egg. When stimulating the ovaries in advance of egg retrieval, you are injecting these hormone signals (that naturally come from the brain to stimulate ovulation of a single egg) – but you are injecting these hormones at a higher level in an effort to select multiple follicles to grow and mature – not just the single one that would normally mature and ovulate naturally. Gonal-F and Follistim Aq are both medications that stimulate the ovaries to produce follicles and release mature eggs and are therapeutic equivalents with MD approval.