|Between puberty and the menopause, a span of some 35 years, a woman ovulates about 400 times, normally at intervals of 28 days. If an egg is fertitized, pregnancy follows; if not, menstruation occurs as the vascular lining of the womb is shed. This remarkable cycle of fertility is maintained by three hormone-producing glands, the hypothalamus, pituitary, and ovaries. The ovaries produce two hormones, oestrogen and progesterone, in response to two hormones produced by the pituitary, follicle stimulating hormone (FSH) and luteinizing hormone (LH). Oestrogen levels, responding to FSH levels, rise dramatically in the 12 days leading up to ovulation, and are at their lowest during menstruation. Levels of progesterone, secreted in response to LH, are highest after ovulation, preparing the lining of the uterus (endometrium) for implantation of the egg. Menstrual bleeding begins approximately 2 days after oestrogen and progesterone secretion reach their lowest level. About 35 ml (1½ fl oz) of blood are then lost over 3-7 days.
The uterus or womb is a thick-walled, pear-shaped organ about 7.5 cm (3 in) long, with a narrow neck or cervix at its lower end; the mouth of the cervix juts into the top of the vagina. Opening into the bulbous, upper end of the uterus are two tubes about 10 cm long (4 in), the Fallopian tubes. These are lined with tiny waving hairs which transport eggs from the ovaries to the uterus. It is in one or other Fallopian tube that fertilization usually occurs. There is no direct connection between the ovaries and the Fallopian tubes, however; eggs are simply released into the fluid-filled peritoneal cavity and wafted towards their fringed entrances.
The ovaries themselves are almond-shaped and about 3.5 cm (1½ in) long. At birth they contain many millions of immature eggs and follicles, but as already mentioned only about 400 reach maturity. Although several eggs may begin to mature each month under the influence of FSH, only one matures. Once it has been released from its follicle, the follicle turns into a 'corpus luteum' (literally 'yellow body') and proceeds to secrete large amounts of progesterone.
The external female genitals, referred to as the vulva, consist of the fleshy inner and outer labia or lips which conceal the entrance to the vagina, and the clitoris, which lies just above the urethral opening where the inner labia meet. The clitoris is very sensitive and a focus for many of the sensations which trigger orgasm. The vagina is about 10 cm (4 in) long, expands and lubricates during sexual arousal, and is most sensitive near its entrance. It is protected from infection by populations of acid-producing bacteria.
Breast development is one of the first signs of puberty. The breasts move with the pectoral muscles underneath and their hang is greatly influenced by posture and the tone of the muscles which brace the shoulders back. Embedded in the fatty tissue of each breast are 15-20 clusters of milk-producing glands whose ducts converge on the nipple. The pigmented area around the nipple, the areola, contains glands which keep the nipple supple.