text: Production of healthy germ cells (sperm & eggs) is regulated by specific hormones (chemical signals) in men and women. Infertility can result if there is imbalance in certain hormones and hence can be treated by external hormone supplements. On the other hand changing the body's natural hormone balance, carefully, can be one of the ways of contraception. \n\nFor a quick definition of hormones: please check answers for the question: functional difference of hormone and enzyme?  By lovec27\n\nAs an example, the events regulated by hormones in a woman are mentioned here: An ovary in a woman's body would produce and release (ovulation) one egg a month and her body prepares (uterine wall thickening etc) to receive and nourish the embryo every month. All these normal events as well as pregnancy require several hormone-mediated changes in a woman's body. Some of these hormones are follicle stimulating hormone (FSH), luteinizing hormone (LH), estrogen (E), progesterone (P). FSH and LH help the eggs grow and when they rise in quantity, E also increases and this causes ovulation. Later P helps the uterus to grow so that, in case there is fertilization, the embryo can settle on uterus wall and eventually grow into a baby. \n\nMale fertility is also regulated by hormones. While hormone-based contraception is available for women, it has not yet reached success for men.