Progesterone (P4 or pregn-4-ene-3,20-dione) is the main progestogenic steroid hormone secreted by the female reproductive system and is connected with the female menstrual cycle, pregnancy and embryogenesis.1
The ovaries, placenta and the adrenal glands produce progesterone to regulate the condition of the endometrium (inner lining) of the uterus.2
The terms progestogen, progesterone and progestin are often confused and used interchangeably. However, to clarify, the terms refer to:3
Progestogen (C-21 steroid hormone) - a generic term for a group of hormones that include progesterone (P4), pregnenolone (P5), 17α-hydroxy-pregnenolone, and 17α-hydroxyprogesterone.
Progesterone - a single chemical entity and the primary progestogenic hormone synthesized by the human body.
Progestin - a term used to refer to synthetic progestational agents.
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Fast facts on progesterone
Here are some key points about progesterone. More detail and supporting information is in the main article.
- Progesterone is secreted by the corpus luteum in the ovary, in the placenta and in the adrenal glands.
- If fertilization does not occur, the corpus luteum breaks down and the secretion of progesterone drops significantly.
- If conception occurs, progesterone prepares the tissue lining of the uterus to accept implantation of the fertilized egg and helps to preserve the endometrium.
- The placenta begins progesterone secretion to supplement that of the corpus luteum during pregnancy.
- If ovulation does not occur, the ovaries will not produce progesterone during that cycle.
- Progesterone levels drop sharply during perimenopause.
- Progesterone travels in the blood to reach tissues where there are progesterone receptors.
- Laboratories can produce progesterone. Synthetic steroid hormones with similar properties are called progestin.
- Men's progesterone levels are comparable to those of women during the follicular phase of the menstruation cycle.
- Static progesterone levels could indicate a problem with ovulation and/or menstruation.
What is progesterone?
After the onset of puberty, the ovaries release a single egg each month, alternating the ovary that releases the egg. This process is called ovulation.1
The egg migrates down the fallopian tube in the hope of meeting with a sperm in order to be fertilized.1
The corpus luteum forms from the empty follicle and is a temporary endocrine gland that develops after ovulation during the luteal phase (latter phase of the menstrual cycle).
The hypothalamus signals to the pituitary gland to release follicle stimulating hormone and luteinizing hormone. These hormones are essential to normal reproductive function and formation of the corpus luteum, which becomes the main source of progesterone required to maintain a pregnancy.
The hypothalamus signals to the pituitary gland to release follicle stimulating hormone (FSH) and luteinizing hormone (LH).
After ovulation, progesterone:
- Prepares the endometrium for potential pregnancy so the lining is able to accept and implant a fertilized egg and support its development
- Inhibits muscle contractions of the uterus to avoid the uterine wall rejecting the adhering egg
- Inhibits development of a new follicle.
If fertilization does not occur and no embryo is conceived, the corpus luteum breaks down and the secretion of progesterone drops significantly to stop the growth of the uterine wall. As the lining of the womb is no longer maintained by progesterone from the corpus luteum, the lining breaks away, resulting in menstrual bleeding.
If conception does occur, progesterone stimulates the further development of blood vessels in the endometrium (the lining of the womb) and stimulates glands in the endometrium to secrete nutrients that nourish the early embryo. Progesterone prepares the tissue lining of the uterus to accept implantation of the fertilized egg and helps to preserve the endometrium throughout pregnancy.5
After conception, the vascular spaces in the endometrium fuse and become interconnected, forming the placenta. The placenta then begins to secrete progesterone to supplement the progesterone already secreted by the corpus luteum.
Levels of progesterone from the placenta remain elevated throughout the pregnancy, and the hormone prevents further eggs from maturing and causes changes in breast tissue to prepare for lactation. Progesterone is vital to the survival of the embryo and fetus throughout gestation and becomes the major hormone supporting the pregnancy.
If a woman fails to ovulate, her ovaries do not produce progesterone; this is referred to as an anovulatory cycle. Missed cycles often take place from mid 30s onwards and become more frequent as perimenopause approaches.6 On average, 52% of cycles are anovulatory by the age of 46.
During perimenopause, estrogen levels can increase by some 20%, become erratic, and then drop significantly at menopause. Progesterone levels drop more consistently during perimenopause and are now thought to be the major cause of symptoms experienced just prior to menopause.
How does progesterone work?
Progesterone works by traveling in the blood to tissues where there are progesterone receptors. Progesterone molecules attach to the receptors and initiate their actions in the body.6
Progesterone has the following effects on the female organs:
- Is responsible (with estrogen) for the release of an egg from the ovaries during ovulation
- Prepares the tissue lining of the uterus to allow the fertilized egg to implant
- Maintains the endometrium throughout pregnancy
- Prevents further egg release until a pregnancy has terminated
- Prevents fertilization of more than one egg at a time - although infrequently more than one egg is released
- Stops muscular contractions in the fallopian tubes once the egg has been transported
- Plays an important role in fetal development during pregnancy
- Stimulates the breast tissue to prevent lactation and causes the glands to advance ready for producing milk
- Strengthens the pelvic wall muscles in preparation for labor.
Progesterone in the menstrual cycle
The female sex hormones control the menstrual cycle and naturally rise and fall throughout the month. The menstrual cycle is a dynamic process that repeats itself every 28 days on average and progesterone plays a leading role in the process.
Day 1: Period12
Estrogen and progesterone levels are at their lowest at this point in the cycle.
Day 5: Egg is selected
Inside the ovary, and egg is selected and the follicle inside which it has grown releases increasing amounts of estrogen.
Days 6-14: Preparing for ovulation
Toward the end of this stage, estrogen levels rise slowly, then more rapidly.
Around Day 14: Ovulation
The follicle surrounding the egg breaks open and the ovary releases the egg into the fallopian tube so it can be fertilized by sperm. The follicle remains in the ovary and turns into the corpus luteum.
Days 15-28: After ovulation
After ovulation has occurred, levels of progesterone start to increase. If the egg that was released is not fertilized, estrogen and progesterone levels drop after approximately 2 weeks and the lining of the uterus gets ready to be shed. The next period begins and the cycle starts again.
On the next page, we look at the role of progesterone in medication.