During puberty, a girl’s complex hormonal system begins to stir to life. These changes cause the development of breasts, feminine curves, and more. It’s around this time that she experiences her first menstrual period, which concludes an approximately 28-day cycle of hormonal interaction readying the body for pregnancy and producing an egg for fertilization. Over the next few decades of her life, she’ll experience this cycle about 450 more times.

The menstrual cycle begins when the brain produces hormones that stimulate the pituitary gland to release gonadotropins, including follicle-stimulating and luteinizing hormones. These bodily chemicals trigger a handful of follicles to begin maturing into eggs. Simultaneously, the hormones also cause the ovaries to begin pumping out estrogen, which stimulates the uterine lining (the endometrium) to thicken in preparation for a fertilized egg. After about 2 weeks, a full-grown egg is released from one of the ovaries into the neighboring fallopian tube, where the egg may join with a sperm. The empty follicle releases progesterone in addition to estrogen, a hormone important to the early stages of pregnancy. If the egg isn’t fertilized, however, the inner lining of the uterus breaks down and is shed from the body during the 3- to 5-day span of menstruation, which occurs about 2 weeks after ovulation. Then the process starts over again.

ADDITIONAL FACTS

  1. The Museum of Menstruation and Women’s Health was located for more than a decade in New Carrollton, Maryland, before it was shuttered. Today, its archive of menstruation history and advertisements lives on its Web site, www.mum.org.
  2. Girls are beginning to menstruate at earlier ages than ever before. Although experts aren’t sure exactly why, one reason may be the increased number of chubby girls; fat makes more estrogen, and girls are reaching the necessary minimum weight for menstruation at earlier ages.
  3. After a girl’s first menstrual period, it takes about 3 years for the menstrual cycle to become regular.