"Both the male and female reproductive systems play a role in pregnancy. Problems with these systems can affect fertility and the ability to have children. Reproductive health problems can also be harmful to overall health and impair a person's ability to enjoy a sexual relationship. Reproductive health is influenced by many factors, including age, lifestyle, habits, genetics, use of medicines, and exposure to chemicals in the environment. Many problems of the reproductive system can be corrected. Reproductive health includes menstruation and menopause, pregnancy and preconception care, fertility and infertility, and contraception. The menstrual cycle is the process by which a woman’s body gets ready for the chance of a pregnancy each month. The average menstrual cycle is 28 days from the start of one period to the start of the next, but it can range from 21 to 35 days in adults and from 21 to 45 days in young teens. Most menstrual periods last from three to five days. In the United States, most girls start menstruating at age 12, but girls can start menstruating between the ages of 8 and 16. Menstruation is a woman's monthly bleeding. During menstruation, the body sheds the lining of the uterus, or womb. Menstrual blood flows from the uterus through the cervix and passes out of the body through the vagina. Having regular menstrual cycles is a sign that important parts of the body are working normally. The menstrual cycle provides hormones to help keep the body healthy and prepares it for pregnancy each month. A cycle is counted from the first day of one period to the first day of the next period. In the first half of the cycle, estrogen levels rise. Estrogen helps build and maintain strong bones and causes the lining of the uterus to grow and thicken. At the same time, an egg in one of the ovaries starts to mature. Around day 14 of an average 28-day cycle, the egg leaves the ovary in a process called ovulation. It travels through the fallopian tube to the uterus. A woman is most likely to become pregnant during the three days before or on the day of ovulation, although women with shorter or longer cycles may ovulate before or after day 14. If the egg is not fertilized, hormone levels drop and the thickened uterine lining is shed during the menstrual period. Day 1 begins with the first day of the period. By about day 7, bleeding has stopped and follicles develop on the ovaries. Between days 7 and 14, one follicle matures while the uterine lining thickens. Around day 14, hormones cause the mature follicle to release an egg. Over the next few days, the egg travels down the fallopian tube. If sperm unites with the egg, the fertilized egg continues to the uterus and attaches to its lining. If the egg is not fertilized, hormone levels drop around day 25, the egg breaks apart, and the next menstrual cycle begins."