Estrogen hormones are the primary female sex hormones. They are responsible for the growth and development of female sexual characteristics and reproduction in both humans and animals. Estrogen is produced in the ovaries, adrenal glands and fat tissues and leads to the development of female secondary sexual characteristics, such as breasts, and are also involved in the thickening of the endometrium and other aspects of regulating the menstrual cycle. During a woman's life, estrogen levels will often fluctuate. They start increasing during puberty and remain at healthy levels until a woman reaches 40s and 50s. During this time, at the start of menopause, estrogen production significantly deceases in a woman's body. Estrogen is also produced in small quantities among males and it is thought that it helps regulate certain functions of the reproductive system important to the maturation of sperm.