Estrogen is a steroid hormone produced by a woman's ovaries. It is a primary sex hormone and is responsible for the functioning of the menstrual cycle. Estrogen receptors on certain cells allow for its diffusion into those cells and is responsible for the development of secondary sexual characteristics at puberty. These include wider hips, breast development and the onset of the menstrual cycle. It allows for the uterine lining to thicken during the course of the menstrual cycle, in antipication of an egg becoming implanted there. It causes an increase in fat storage as well as metabolic rate. Estrogen aids in the formation of bone, lubrication of the vagina during intercourse, regulates the HDL and LDL balance in the body and regulates fluid balance. Estrogen when it surges causes another hormone called luteinizing hormone to trigger ovulation each month. Estrogen levels affect sexual desire and in other mammals, it causes mating desire and affects the estrus cycle. Estrogen plays a role in protecting against heart disease, and it also affects mental health in females.