The uterus is a pear-shaped organ located between the bladder and lower intestine. It consists of two parts, the body and the cervix. When a woman is not pregnant the body of the uterus is about the size of a fist, with its walls collapsed and flattened against each other. During pregnancy the walls of the uterus are pushed apart as the fetus grows. The cervix is the lower third of the uterus; it has a canal opening into the vagina, with an opening called the os, which allows menstrual blood to flow out of the uterus into the vagina. Leading off each side of the body of the uterus are two tubes, known as the fallopian tubes. Near the end of each tube is an ovary. Ovaries are egg-producing organs that hold between 200,000 and 400,000 follicles (from folliculus, meaning "sack" in Latin); these cellular sacks contain the materials needed to produce ripened eggs, or ova. The inner lining of the uterus is called the endometrium, and during pregnancy it thickens and becomes enriched with blood vessels to house and support the growing fetus. If pregnancy does not occur, the endometrium is shed as part of the menstrual flow. Menstrual flow also consists of blood and mucus from the cervix and vagina.

Reproductive Hormones

The hypothalamus (an area in the brain) and the pituitary gland regulate the reproductive hormones. The pituitary gland is often known as the master gland because of its important role in many vital functions, many of which require hormones. In women, six key hormones serve as chemical messengers that regulate the reproductive system. The hypothalamus first releases the gonadotropin-releasing hormone (GnRH). This chemical, in turn, stimulates the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Estrogen, progesterone, and the male hormone testosterone are secreted by the ovaries at the command of FSH and LH and complete the hormonal group necessary for reproductive health.