Hormones are produced in small organs called glands that are located inside your body and brain. When your body is ready to enter puberty, a gland called the pituitary releases hormones that set the process in motion. This tiny pea-shaped organ is located at the base of your brain, in an area called the hypothalamus. The beginning of puberty is a bit like a row of dominoes falling. First, the hypothalamus starts producing a hormone called gonadotropin-releasing hormone (GnRH), which develops in your blood. When its levels are high enough, GnRH signals the pituitary to release two other hormones called follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH travel through the blood and reach your ovaries (see page 36) to tell them to start producing hormones called estrogen and progesterone.

Together, FSH, LH, estrogen, and progesterone cause the physical changes your body will undergo during puberty. They also regulate your menstrual cycle. The levels of these hormones will continue to fluctuate in cycles for as long as you still have your period. For most women, this will continue until they go through menopause, after the age of fifty.