Estrogen is one of the key sex hormones that contributes to the development and regulation of the female reproductive system and secondary sex characteristics. There are three major types of estrogen - estrone, estradiol, and estriol. Estrone is the weakest form and levels decrease after menopause. Estradiol is the strongest and most abundant estrogen during reproductive years. Estriol is the primary estrogen produced during pregnancy. All three help develop and maintain female reproductive tissues. Estrogen plays a huge role in the menstrual cycle. In the first half of the cycle or follicular phase, rising estrogen levels stimulate follicle growth and thickening of the uterine lining. This peaks right before ovulation. After ovulation in the luteal phase, estrogen works together with progesterone for functions like preparing the body for pregnancy. If no pregnancy occurs, it drops causing menstruation. Throughout a woman's life, balanced estrogen levels are essential for many aspects of health like: