Sex hormones and secondary sexual characteristics at puberty

During puberty, the body produces more sex hormones. These hormones stimulate the development of secondary sexual characteristics: physical features associated with sexual maturity that are not the reproductive organs themselves.

Testosterone and physical changes

Testosterone, produced mainly by the testes, promotes changes including a deeper voice, increased facial and body hair, and greater muscle development.

Oestrogen and physical changes

Oestrogen, produced mainly by the ovaries, promotes changes including breast development and a wider distribution of body fat around the hips.

Sex hormones also help maintain the secondary sexual characteristics that develop.

Ovarian and uterine changes during the menstrual cycle

The menstrual cycle is a recurring sequence of linked changes in the ovaries and the lining of the uterus. While an egg matures in an ovary, the uterus prepares for a possible pregnancy.

A cycle begins with menstruation: the uterine lining breaks down and is shed. After menstruation, the lining rebuilds and becomes thicker. Meanwhile, an egg matures in an ovary. The egg is then released from the ovary in a process called ovulation.

The thickened uterine lining provides a place where a pregnancy can develop. If pregnancy does not occur, the lining breaks down and is shed during the next menstruation. This begins a new cycle.

Hormonal control of the menstrual cycle

Hormones coordinate the development and release of an egg with changes in the lining of the uterus. FSH and LH are released by the pituitary gland and act on the ovaries. The ovaries produce oestrogen and progesterone, which act on the uterine lining and affect the release of FSH and LH.

Follicle development and ovulation

Follicle-stimulating hormone (FSH) stimulates an ovarian follicle to develop. A follicle is a structure in the ovary that contains an immature egg. As the follicle develops, it produces oestrogen.

Oestrogen stimulates the uterine lining to rebuild and thicken after menstruation. A rise in oestrogen also helps trigger a surge in luteinising hormone (LH). This surge causes ovulation: the release of an egg from the follicle.

The corpus luteum and menstruation

After ovulation, LH promotes the formation of the corpus luteum from the emptied follicle. The corpus luteum produces progesterone, which maintains the thickened uterine lining.

Oestrogen and progesterone also inhibit the release of FSH and LH. This limits the development of further follicles and the release of further eggs during that cycle. Oestrogen therefore has different effects at different stages: its rise helps trigger the LH surge before ovulation, while oestrogen and progesterone later inhibit FSH and LH release.

If pregnancy does not occur, the corpus luteum breaks down. Progesterone and oestrogen levels fall, and the uterine lining is shed during menstruation.

Hormonal maintenance of pregnancy

During pregnancy, the hormones progesterone and oestrogen help maintain the uterus and prevent another ovulation. Their production continues throughout pregnancy, although their main source changes.

In early pregnancy, the corpus luteum in the ovary continues to produce both hormones. Later, the placenta becomes the main site of progesterone and oestrogen production.

Maintaining the uterus

Progesterone maintains the thickened uterine lining, preventing it from being shed. Oestrogen also supports the maintenance of the uterus during pregnancy.

Preventing another ovulation

Sustained levels of progesterone and oestrogen inhibit the release of follicle-stimulating hormone (FSH) and luteinising hormone (LH). Without the hormonal stimulation needed for another ovarian follicle to develop and ovulate, another egg is not released while the pregnancy continues.