quality:correction, structural:scholar_edition
Cycles and phases

The menstrual cycle encompasses the ovarian and uterine cycles. The ovarian cycle describes changes that occur in the follicles of the ovary, whereas the uterine cycle describes changes in the endometrial lining of the uterus. Both cycles can be divided into phases. The ovarian cycle consists of alternating follicular and luteal phases, and the uterine cycle consists of menstruation, the proliferative phase, and the secretory phase. The menstrual cycle is controlled by the hypothalamus in the brain, and the anterior pituitary gland at the base of the brain. The hypothalamus releases gonadotropin-releasing hormone (GnRH), which causes the nearby anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Before puberty, GnRH is released in low steady quantities and at a steady rate. After puberty, GnRH is released in large pulses, and the frequency and magnitude of these determine how much FSH and LH are produced by the pituitary.

Measured from the first day of one menstruation to the first day of the next, the length of a menstrual cycle varies but has a median length of 28 days. The cycle is often less regular at the beginning and end of a woman's reproductive life. At puberty, a child's body begins to mature into an adult body capable of sexual reproduction; the first period (called menarche) occurs at around 12 years of age and continues for about 30–45 years. Menstrual cycles end at menopause, which is usually between 45 and 55 years of age.