What is progesterone?

Progesterone belongs to a group of steroid hormones called ‘progestogens’ (which are hormones that have a similar action to the natural hormone progesterone). Synthetic hormones that have a similar action to progesterone are called ‘progestins’. Progesterone is mainly secreted by the corpus luteum in the ovary during the second half of the menstrual cycle. It plays an important role in the menstrual cycle and in maintaining the early stages of pregnancy.

During the menstrual cycle, when an egg is released from the ovary at ovulation (approximately day 14), the remnants of the ovarian follicle that enclosed the developing egg form a structure called the ‘corpus luteum’, which literally translates as ‘yellow body’ due to its appearance. This releases progesterone and, to a lesser extent, oestradiol. The progesterone prepares the body for pregnancy in the event that the released egg is fertilised. If the egg is not fertilised, the corpus luteum breaks down, the production of progesterone falls and a new menstrual cycle begins.

If the egg is fertilised, progesterone stimulates the growth of blood vessels that supply the lining of the womb (endometrium) and stimulates glands in the endometrium to secrete nutrients that nourish the early embryo. Progesterone then prepares the tissue lining of the uterus to allow the fertilised egg to implant and helps to maintain the endometrium throughout pregnancy. During the early stages of pregnancy, progesterone is still produced by the corpus luteum and is essential for supporting the pregnancy and establishing the placenta. Once the placenta is established, it then takes over progesterone production at around weeks 8-12 of pregnancy ‘luteo-placental shift’. During pregnancy, progesterone plays an important role in the development of the foetus (it stimulates the growth of maternal breast tissue; prevents lactation; and strengthens the pelvic wall muscles in preparation for labour). The level of progesterone in the body steadily rises throughout pregnancy until labour occurs and the baby is born.

Although the corpus luteum in the ovaries is the major site of progesterone production in humans, progesterone is also produced in smaller quantities by the ovaries themselves, the adrenal glands and, during pregnancy, the placenta.

How is progesterone controlled?

The formation of the corpus luteum (which produces the majority of progesterone) is triggered by a surge in luteinising hormone production by the anterior pituitary gland. This normally occurs at approximately day 14 of the menstrual cycle and it stimulates the release of an egg from the ovary (ovulation) and the formation of the corpus luteum from the remnant of the follicle. The corpus luteum then secretes progesterone, which prepares the body for pregnancy. If the egg is not fertilised and no embryo is conceived, the corpus luteum breaks down and the production of progesterone decreases. As the lining of the womb is no longer maintained by progesterone from the corpus luteum, it breaks away and menstrual bleeding occurs, marking the start of a new menstrual cycle.

However, if the ovulated egg is fertilised and gives rise to an embryo, the cells that surround this early embryo (which are destined to form the placenta) will secrete human chorionic gonadotrophin. This hormone has a very similar chemical structure to luteinising hormone. This means it can bind to and activate the same receptors as luteinising hormone, meaning that the corpus luteum does not break down and instead keeps producing progesterone until the placenta is established.

What happens if I have too much progesterone?

There are no known serious medical consequences due to the body making too much progesterone. Levels of progesterone do increase naturally in pregnancy as mentioned above. High levels of progesterone are associated with the condition congenital adrenal hyperplasia. However, the high progesterone levels are a consequence of and not a cause of this condition.

Taking high levels of progesterone in the form of medication can be associated with a small increased risk for developing breast cancer.

Progesterone, either alone or in combination with oestrogen, is taken by women as an oral contraceptive ('the pill'). 'The pill' works by preventing ovulation, as well as thickening cervical mucus, making it effective in preventing pregnancy.

Progesterone is used in hormone replacement therapy to relieve symptoms of the menopause in women. There are many recognised pros and cons to hormone replacement therapy – see the article on menopause for more information.

What happens if I have too little progesterone?

If progesterone is absent or levels are too low, irregular and heavy menstrual bleeding can occur. A drop in progesterone during pregnancy can result in a miscarriage and early labour. Mothers at risk of giving birth too soon can be given a synthetic form of progesterone to delay the onset of labour.

Lack of progesterone in the bloodstream can mean the ovary has failed to release an egg at ovulation, as can occur in women with polycystic ovary syndrome.