Is a sex hormone involved in the menstrual cycle, pregnancy, and the development of embryos. Produced following ovulation it plays a key role in embryo implantation and also helps to maintain a pregnancy. It is usual to take progesterone supplements during IVF as other drugs given during IVF and the egg retrieval process can decrease progesterone-producing cells.

Progesterone is an endogenous steroid (a steroid originating from within the cell) and sex hormone belonging to a group of steroid hormones called “progestogens”. Synthetic hormones that have a similar action to progesterone are known as “progestins”. The hormone itself is mostly secreted by the corpus luteum (a temporary gland formed after a follicle has released an egg) in the ovary, during the second half of a menstrual cycle. It also plays an important role in the menstrual cycle itself, as well as in maintaining the early stages of pregnancy.

To understand more about this, it is important to know what is happening in the body around this time. When an egg is released from the body as part of ovulation, around Day 14 of a person’s cycle, the remnants of the follicle that held that egg will form a structure called the corpus luteum. It releases progesterone and oestradiol (a type of oestrogen). 

When progesterone is released by the corpus luteum, the hormone’s first function will be to prepare the body for pregnancy. This is done if the released egg is fertilised. If the egg is not fertilised, the corpus luteum will break down, the production of progesterone will decline, and a new menstrual cycle will begin.

If the egg is fertilised by a sperm cell, progesterone will instead stimulate the growth of blood vessels that supply the uterus (womb) lining. It will also stimulate glands within the uterus lining itself to secrete nutrients that will nourish the embryo at its earliest stages. Progesterone will also prepare the tissue lining of the uterus to allow the fertilised egg to implant while helping to maintain the uterus throughout the pregnancy. 

During the early stages of pregnancy, progesterone will still mostly be produced by the corpus luteum. This makes it essential for supporting the pregnancy and establishing the placenta. Once the placenta has been established, it will then take over the role of the corpus luteum and produce progesterone. This is known as the “luteo-placental shift”. 

Progesterone plays an extended role in the development of the foetus, by:

  • Stimulating the growth of maternal breast tissue
  • Preventing lactation
  • Strengthening the pelvic wall muscles, in preparation for labour

The level of progesterone in the body will steadily rise throughout pregnancy until labour occurs and the baby is born. 

There are no known serious medical consequences of having too much progesterone in the body. High levels of the hormone are associated with the condition congenital adrenal hyperdysplasia, but studies have shown that the hormone increase is a consequence of the condition, not a cause. Progesterone is also taken regularly, along with oestrogen, as part of the combined contraceptive pill.

If progesterone is absent from the body, or levels of it are too low, there is a chance that this may result in heavy or irregular menstrual bleeding. It can also mean that the ovary has failed to release an egg at ovulation, which may occur if the person has polycystic ovary syndrome (PCOS).