Progesterone is a steroid hormone involved in the female reproductive system. Instead of originating from a single “progesterone gland,” its production is complex and comes from different sources depending on the body’s needs. This system involves multiple structures that work at different times to regulate the hormone’s presence.
The Primary Sources of Progesterone
The main sites of progesterone production in the female body are temporary and cyclical. Following ovulation, which typically occurs around the midpoint of the menstrual cycle, the remnant of the ovarian follicle transforms into a temporary endocrine gland known as the corpus luteum. This structure becomes the principal producer of progesterone for the second half of the cycle, a period called the luteal phase. The hormone is synthesized from cholesterol under the influence of luteinizing hormone (LH).
This production by the corpus luteum prepares the body for a potential pregnancy. If fertilization does not occur, the corpus luteum breaks down, leading to a sharp decline in progesterone levels and initiating menstruation. If fertilization and implantation happen, the corpus luteum continues its progesterone output to support the early stages of pregnancy.
As a pregnancy progresses, progesterone production shifts. Around the eighth to twelfth week of gestation, the placenta takes over as the primary source. This event is sometimes called the luteo-placental shift. The placenta synthesizes increasing amounts of the hormone throughout the pregnancy, with levels peaking during the third trimester.
Secondary and Male Sources of Progesterone
Progesterone is also synthesized in smaller quantities in other areas in both females and males. The adrenal glands, located on top of the kidneys, produce progesterone in both sexes. In this context, progesterone acts as a precursor molecule, meaning it is a substance from which other hormones are made. The adrenal glands convert it into other steroid hormones, including cortisol and aldosterone.
In males, the testes also produce a small amount of progesterone. It serves as an intermediate in the biochemical pathway for producing testosterone. The amount produced in the testes is minor compared to the levels seen in females during the menstrual cycle or pregnancy.
The Role of Progesterone in the Body
The primary function of progesterone is to prepare the uterus for and maintain a pregnancy. After ovulation, rising levels of the hormone act on the uterine lining, known as the endometrium. It causes the endometrium to thicken and develop a rich network of blood vessels, creating a nourishing environment for a fertilized egg to implant and grow.
Once an embryo has implanted, progesterone continues to play a supportive role. It helps maintain the pregnancy by preventing uterine muscle contractions that could disrupt the early embryo. Progesterone also stimulates glands within the endometrium to secrete nutrients that nourish the developing embryo before the placenta is fully formed. Throughout gestation, it contributes to the growth of maternal breast tissue for lactation.
Progesterone also works with estrogen to regulate the menstrual cycle. During the luteal phase, its presence helps stabilize the uterine lining. This cyclical rise and fall of hormones are what drives the regular rhythm of the menstrual cycle.