The ovaries get not one but two different hormonal signals from the pituitary gland, which lies deep in the brain. The pituitary releases both FSH (follicle stimulating hormone) and LH (luteinizing hormone). Both hormones have multiple functions in the body. In the ovaries, the primary job of FSH is to stimulate the creation of a follicle, something like a rope, pulling the egg out of the ovary. LH dissolves the rope to allow the egg to be released during ovulation. But the pituitary doesn't act on its own.
The pituitary is stimulating the pulses of gonadotrophin releasing hormone, or GnRH. This hormone comes from the hypothalamus. And the hypothalamus itself responds to increases and decreases in estrogen levels that give it feedback on the progress of the menstrual cycle. If any part of this elaborate control system goes wrong, there won't be ovulation. If there is damage to the pituitary or the hypothalamus, there may not be ovulation. If the ovaries don't respond to hormones, there may not be ovulation. If the follicle does not form at the right time or dissolve at the right time, there may not be ovulation. And even if there is ovulation, there has to be fertilization, and the uterus has to be ready to receive the fertilized egg.
Many things can go wrong. Amazingly, sometimes just going on a high-protein, reduced-calorie PCOS diet is enough to fix them. The average figure in the medical literature is 50 per cent, but anywhere from 44 to 92 per cent of women who have PCOS can conceive within six months to a year of losing just 5 to 10 per cent of their total weight. It is not necessary to do both exercise and diet, although moderate (not severe) exercise may increase the frequency of ovulation. Just losing a little weight usually is enough to cause a favorable change in hormonal balance that leads to conception. There are many, many women with PCOS who will still need medication, ovarian drilling, or even in vitro fertilization, but diet alone is very often the key to the ability to conceive.