- Progesterone is a steroid hormone that plays a central role in the female reproductive system, particularly in regulating the menstrual cycle, supporting pregnancy, and preparing the body for conception.
- It is produced primarily by the corpus luteum in the ovary after ovulation during the second half of the menstrual cycle (the luteal phase), and later by the placenta during pregnancy. Smaller amounts of progesterone are also synthesized in the adrenal glands and, in males, in the testes. As a key progestogen, progesterone orchestrates a wide array of physiological functions in the reproductive tract, central nervous system, and other tissues.
- During the menstrual cycle, after an egg is released from the ovary, the ruptured follicle transforms into the corpus luteum, which begins secreting progesterone. This hormone acts on the endometrium (the lining of the uterus) to stimulate its thickening and vascularization, making it receptive to a potential embryo. If fertilization and implantation occur, progesterone continues to support the endometrium and prevents its breakdown. If pregnancy does not occur, progesterone levels fall, leading to the shedding of the uterine lining—menstruation.
- In early pregnancy, progesterone is essential for maintaining the uterine environment. It promotes the growth of blood vessels in the endometrium, inhibits uterine contractions to prevent premature expulsion of the embryo, and helps modulate the maternal immune response to tolerate the semi-allogeneic fetus. Around the end of the first trimester, the placenta takes over progesterone production, ensuring a steady supply throughout gestation. Without adequate progesterone, pregnancy cannot be sustained, which is why it is sometimes supplemented in assisted reproductive technologies (ART) or in women with recurrent pregnancy loss.
- Beyond reproduction, progesterone has several systemic effects. It influences the central nervous system, acting as a neurosteroid with calming, sedative-like properties and modulating mood and cognition. It also plays a role in breast development, preparing the mammary glands for milk production in conjunction with estrogen. Progesterone affects the respiratory system, increasing sensitivity to carbon dioxide and promoting deeper breathing during pregnancy. It also alters renal function, metabolism, and even skin elasticity, highlighting its widespread physiological influence.
- Progesterone levels are tightly regulated and fluctuate throughout the menstrual cycle and pregnancy. Measuring serum progesterone can provide valuable clinical information, such as confirming ovulation or evaluating luteal phase sufficiency. Abnormally low levels may indicate luteal phase defect, anovulation, or impending miscarriage, while elevated levels may suggest ovarian cysts, molar pregnancy, or adrenal disorders.
- In clinical practice, synthetic and natural forms of progesterone (progestins) are widely used in hormonal contraceptives, hormone replacement therapy (HRT), and treatments for menstrual disorders such as endometriosis, premenstrual syndrome (PMS), and abnormal uterine bleeding. Natural progesterone is often preferred in HRT for its favorable profile in terms of cardiovascular risk and breast health compared to some synthetic progestins.
