Progesterone

Progesterone is a steroid hormone with central roles in the ovarian cycle, menstrual cycle, implantation, pregnancy, and mammary gland development. In the nonpregnant reproductive cycle, its major source is the corpus luteum, a temporary endocrine structure formed from the ovarian follicle after ovulation. During pregnancy, the placenta becomes a major source of progesterone.

Progesterone acts primarily on tissues that have been prepared by estrogen, particularly the endometrium. After ovulation, rising progesterone transforms the estrogen-stimulated proliferative endometrium into a secretory endometrium capable of supporting implantation and early embryonic development. Progesterone also modifies cervical mucus, influences the uterine tubes, decreases myometrial excitability, contributes to breast development, and provides negative feedback within the hypothalamic-pituitary-gonadal axis.

Because progesterone concentrations change dramatically across the ovarian cycle, the hormone serves as one of the major endocrine signals distinguishing the follicular and luteal phases.

Hormone Classification

Progesterone is a steroid hormone derived from cholesterol. It belongs to the class of steroid hormones known as progestogens.

Like other steroid hormones, progesterone is lipid soluble and can cross cell membranes to interact primarily with intracellular receptors.

Major Sources of Progesterone

Source	Physiological Context
Corpus luteum	Major source during the luteal phase of the ovarian cycle
Placenta	Major source during much of pregnancy
Ovarian follicles	Small amounts produced during follicular steroidogenesis
Adrenal glands	Small quantities and steroidogenic intermediate

Ovarian Production

The ovary becomes a major source of progesterone after ovulation. Before ovulation, progesterone concentrations are relatively low compared with those of the luteal phase.

The dramatic postovulatory rise occurs when cells of the ruptured follicle undergo luteinization and form the corpus luteum.

Ovulation and Luteinization

Ovulation is triggered by the midcycle LH surge. Following rupture of the mature follicle and release of the secondary oocyte, the remaining granulosa and theca cells undergo structural and functional transformation.

This process, called luteinization, converts the postovulatory follicle into a highly vascular endocrine structure specialized for steroid hormone secretion.

Corpus Luteum

The corpus luteum is a temporary ovarian endocrine gland formed after ovulation. It occupies the site of the ruptured follicle and becomes the dominant source of ovarian progesterone during the luteal phase.

It also produces estradiol, inhibin A, and other regulatory factors.

Granulosa Lutein Cells

After ovulation, granulosa cells enlarge and differentiate into granulosa lutein cells. These cells are major contributors to progesterone production within the corpus luteum.

Luteinization changes their steroidogenic enzyme expression and allows substantial progesterone secretion.

Theca Lutein Cells

Cells derived from the theca interna differentiate into theca lutein cells. They occupy a more peripheral position within the corpus luteum and contribute to luteal steroidogenesis.

Interactions between granulosa lutein and theca lutein cells support the endocrine function of the corpus luteum.

LH and Progesterone Production

Luteinizing hormone (LH) is essential for normal corpus luteum formation and function during the nonpregnant ovarian cycle.

LH receptor signaling promotes steroidogenesis and supports progesterone secretion by luteal cells.

Synthesis From Cholesterol

Progesterone is synthesized from cholesterol. Cholesterol is transported into mitochondria, where the steroidogenic pathway begins.

The first major steroid product is pregnenolone, which is subsequently converted to progesterone.

Basic Progesterone Synthesis

Step	Process
1	Cholesterol is made available to steroidogenic cells
2	Cholesterol is transported into mitochondria
3	Cholesterol is converted to pregnenolone
4	Pregnenolone is converted to progesterone
5	Progesterone diffuses from the steroidogenic cell into the circulation

Pregnenolone

Pregnenolone is produced from cholesterol and serves as an important precursor for steroid hormone synthesis.

It can be converted into progesterone by the action of 3β-hydroxysteroid dehydrogenase and associated steroidogenic machinery.

Progesterone as a Steroid Precursor

Progesterone is not only a biologically active hormone. It also occupies an important position within steroidogenic pathways leading to glucocorticoids, mineralocorticoids, and androgens in appropriate endocrine tissues.

The final products generated from steroid precursors depend on the enzymes expressed by each steroidogenic tissue.

Progesterone During the Follicular Phase

Circulating progesterone concentrations are relatively low during most of the follicular phase. Developing follicles are primarily characterized by increasing estrogen production.

A modest increase in progesterone production occurs around the periovulatory period as the mature follicle begins luteinization-related changes.

Progesterone During the Luteal Phase

After ovulation, progesterone concentrations rise substantially as the corpus luteum develops.

The luteal phase is therefore the progesterone-dominant portion of the ovarian cycle.

Peak Luteal Progesterone

Progesterone generally reaches high concentrations during the midluteal phase when the corpus luteum is maximally functional.

This hormonal environment corresponds with maximal secretory transformation of the endometrium.

Progesterone Across the Ovarian Cycle

Cycle Stage	Progesterone Pattern	Major Source
Early follicular phase	Low	Minimal ovarian secretion
Late follicular phase	Generally low with periovulatory increase	Mature follicle
Early luteal phase	Rapidly rising	Developing corpus luteum
Midluteal phase	High	Functional corpus luteum
Late luteal phase without pregnancy	Falls	Regressing corpus luteum
Pregnancy	Maintained and progressively supported by placental production	Corpus luteum initially, then placenta becomes the major source

Endometrial Effects

The endometrium is one of the most important progesterone target tissues. During the follicular phase, estrogen stimulates proliferation of the functional endometrial layer.

After ovulation, progesterone acts on this estrogen-primed tissue and converts it from a proliferative state into a secretory state.

Secretory Transformation

Progesterone promotes the secretory transformation of endometrial glands. The glands become increasingly tortuous and develop secretory activity.

The endometrial stroma and vascular environment also undergo changes that prepare the uterus for possible implantation.

Secretory Phase

The uterine secretory phase corresponds approximately to the ovarian luteal phase. It begins after ovulation under the influence of progesterone from the corpus luteum.

The functional endometrium becomes structurally and biochemically prepared for interaction with an implanting blastocyst.

Endometrial Glands

Progesterone stimulates secretory changes within endometrial glands that had proliferated under estrogenic influence.

These glands provide a nutrient-rich environment during the period when an early conceptus may enter the uterine cavity and begin implantation.

Endometrial Stroma

Progesterone also affects stromal cells of the endometrium. During the late luteal phase and particularly during pregnancy, stromal cells can undergo decidualization.

Decidualized cells form an important component of the maternal tissue surrounding an implanted embryo.

Decidualization

Decidualization is the transformation of endometrial stromal cells into specialized decidual cells under hormonal influence, particularly progesterone.

This process is essential for normal implantation, placental development, and regulation of the maternal-fetal interface.

Progesterone and Implantation

Successful implantation requires an appropriately prepared endometrium. Progesterone creates the postovulatory endometrial environment necessary for uterine receptivity.

Its actions occur in coordination with preceding estrogen exposure and numerous local signaling molecules.

Window of Implantation

The endometrium becomes optimally receptive to implantation during a limited period of the secretory phase. This period is commonly referred to as the window of implantation.

Progesterone-dependent changes in epithelial, glandular, stromal, vascular, and molecular characteristics contribute to this receptive state.

Progesterone Withdrawal

If pregnancy does not occur, the corpus luteum undergoes luteolysis and progesterone concentrations fall.

Progesterone withdrawal is a major endocrine event leading to breakdown and shedding of the functional endometrial layer during menstruation.

Menstruation

The decline in progesterone and estrogen after corpus luteum regression triggers molecular and vascular changes within the endometrium.

These changes ultimately result in shedding of the functional layer and menstrual bleeding.

Spiral Arteries

The functional endometrium is supplied by spiral arteries. During the luteal phase, the endometrial vascular system undergoes progesterone-dependent changes as the tissue prepares for implantation.

When ovarian steroid support is withdrawn, vascular and inflammatory processes contribute to menstrual breakdown.

Progesterone and the Myometrium

Progesterone contributes to maintenance of a relatively quiescent uterine environment. It reduces myometrial excitability and modifies signaling pathways involved in uterine contractility.

This function becomes especially important during pregnancy.

Uterine Contractility

The myometrium is the smooth muscle layer of the uterus. Its contractile state is influenced by multiple hormones, receptors, ion channels, and local signaling molecules.

Progesterone favors mechanisms that limit excessive uterine contractions during pregnancy.

Progesterone and Oxytocin Responsiveness

Progesterone influences the uterine signaling environment associated with contractility, including pathways related to oxytocin responsiveness.

Near parturition, changes in the balance of progesterone-dependent and estrogen-dependent signaling contribute to increased uterine responsiveness.

Progesterone and the Cervix

Progesterone influences the properties of cervical mucus. After ovulation, cervical mucus generally becomes thicker and less favorable to sperm penetration than the thin, more permeable mucus associated with high preovulatory estrogen.

This cyclic change reflects the transition from estrogen dominance to progesterone dominance.

Progesterone and the Uterine Tubes

Progesterone influences secretory and muscular activity within the uterine tubes. Together with estrogen, it helps regulate the tubal environment encountered by gametes and the early embryo.

Progesterone and the Vagina

Progesterone contributes to cyclic changes in the reproductive tract, including changes in vaginal epithelial characteristics.

Its effects occur within the broader hormonal environment created by estrogen and other reproductive signals.

Progesterone and the Breast

Progesterone contributes to development and differentiation of the mammary glands. It acts particularly on the lobuloalveolar structures of the breast.

Its effects occur together with estrogen, prolactin, growth hormone, and other hormonal signals.

Lobuloalveolar Development

Estrogen is particularly important for ductal development, while progesterone contributes substantially to development of the lobules and alveolar structures associated with secretory function.

During pregnancy, extensive mammary differentiation occurs under the combined influence of multiple hormones.

Progesterone and the HPG Axis

Progesterone participates in feedback regulation of the hypothalamic-pituitary-gonadal axis.

During the luteal phase, progesterone acts together with estrogen and inhibin to suppress gonadotropin secretion and prevent inappropriate initiation of another follicular cycle.

Hypothalamic Effects

Progesterone influences hypothalamic regulation of gonadotropin-releasing hormone (GnRH). One important effect is modification of the frequency and pattern of GnRH pulsatility.

These changes contribute to the characteristic gonadotropin environment of the luteal phase.

Pituitary Effects

Progesterone contributes to negative feedback at the pituitary as part of the luteal endocrine environment.

The combined effects of progesterone, estradiol, and inhibin maintain relatively suppressed gonadotropin secretion while the corpus luteum remains active.

Progesterone and LH

LH is required for normal luteal function, while progesterone produced by the corpus luteum contributes to feedback that modifies subsequent LH secretion.

This creates a regulatory relationship between pituitary gonadotropin output and ovarian luteal activity.

Progesterone and FSH

Progesterone contributes to the hormonal environment suppressing FSH during the luteal phase. Estradiol and inhibin A also participate importantly in this suppression.

When the corpus luteum regresses, falling progesterone, estrogen, and inhibin permit FSH to begin rising again.

Luteolysis

Luteolysis is the structural and functional regression of the corpus luteum when pregnancy does not occur.

As luteal cells lose steroidogenic function, circulating progesterone concentrations decline sharply.

Corpus Albicans

Following luteolysis, the corpus luteum is progressively replaced by fibrous scar tissue known as the corpus albicans.

This marks the end of the endocrine function of that particular postovulatory follicle.

Progesterone if Pregnancy Occurs

If fertilization and implantation occur, progesterone production must be maintained because premature loss of luteal progesterone would compromise the early pregnancy-supporting endometrium.

The developing conceptus provides an endocrine signal that rescues the corpus luteum from its normal cyclic regression.

Human Chorionic Gonadotropin

Human chorionic gonadotropin (hCG) is produced initially by trophoblastic tissue associated with the developing conceptus. Its structure and receptor activity allow it to stimulate the LH receptor.

hCG therefore maintains corpus luteum steroidogenesis during early pregnancy.

Corpus Luteum of Pregnancy

The rescued corpus luteum, often called the corpus luteum of pregnancy, continues to produce progesterone during early gestation.

This maintains endometrial support while placental steroidogenic capacity develops.

Luteal-Placental Transition

As pregnancy progresses, the placenta becomes capable of producing sufficient progesterone to assume the major role previously performed by the corpus luteum.

This shift is commonly referred to as the luteal-placental transition.

Placental Progesterone

The placenta becomes a major source of progesterone during pregnancy. Placental trophoblast cells use maternal cholesterol as an important substrate for steroid synthesis.

Progesterone produced by the placenta enters both maternal and fetal circulations and supports the hormonal environment of pregnancy.

Progesterone During Pregnancy

Progesterone has multiple roles during pregnancy. It supports the decidualized endometrium, contributes to uterine quiescence, participates in immunological regulation at the maternal-fetal interface, and supports mammary gland development.

Its actions are integrated with those of estrogens, hCG, human placental lactogen, prolactin, and numerous local mediators.

Maintenance of the Decidua

The decidua is the specialized endometrium of pregnancy. Progesterone is essential for maintaining its differentiated state and supporting the maternal environment surrounding the implanted conceptus.

Maternal-Fetal Interface

Progesterone contributes to regulation of cellular and molecular processes at the maternal-fetal interface. These mechanisms help create an environment compatible with continued pregnancy.

Pregnancy maintenance, however, depends on a complex network of endocrine, immune, vascular, and placental factors rather than progesterone alone.

Progesterone and Parturition

Parturition requires a transition from uterine quiescence to coordinated myometrial contractility. In humans, this transition involves changes in progesterone signaling rather than simply a dramatic fall in circulating progesterone before labor.

Alterations in progesterone receptor function, estrogenic signaling, prostaglandins, oxytocin responsiveness, and inflammatory pathways contribute to the onset of labor.

Functional Progesterone Withdrawal

The term functional progesterone withdrawal describes reduced progesterone-mediated restraint of uterine activation despite continued relatively high circulating progesterone concentrations near term.

Changes in receptor isoforms and downstream signaling are among the mechanisms involved.

Progesterone Receptors

Progesterone exerts many of its actions through progesterone receptors (PRs), members of the nuclear receptor superfamily.

Important classical receptor isoforms include PR-A and PR-B.

PR-A and PR-B

PR-A and PR-B arise from the same gene but differ in their structure and transcriptional activity.

The relative expression and activity of these receptor isoforms can influence tissue-specific responses to progesterone.

Genomic Progesterone Signaling

Progesterone diffuses across the plasma membrane and binds intracellular progesterone receptors. Ligand binding alters receptor conformation and allows regulation of gene transcription.

The resulting changes in gene expression contribute to the relatively sustained actions of progesterone in target tissues.

Progesterone Response Elements

Activated progesterone receptors interact with regulatory DNA sequences and transcriptional cofactors to modify expression of progesterone-responsive genes.

The genes affected differ among the uterus, breast, brain, and other target tissues.

Non-Genomic Progesterone Effects

Progesterone can also produce relatively rapid cellular effects through mechanisms that do not depend exclusively on classical nuclear transcription.

Membrane-associated progesterone signaling contributes to the diversity of progesterone responses in different tissues.

Estrogen and Progesterone Receptors

Estrogen increases progesterone receptor expression in several reproductive tissues. Consequently, estrogen exposure during the follicular phase helps prepare tissues to respond to progesterone after ovulation.

This is an important example of hormonal interaction within reproductive physiology.

Progesterone and Estrogen

Estrogen and progesterone frequently produce sequential and complementary effects. Estrogen promotes proliferation, while progesterone often promotes differentiation of estrogen-primed reproductive tissues.

The endometrium provides the clearest example of this relationship.

Endometrial Sequence

Stage	Dominant Hormonal Influence	Endometrial Response
Menstrual phase	Withdrawal of ovarian steroids	Shedding of functional layer
Proliferative phase	Estrogen	Regrowth and proliferation
Secretory phase	Progesterone after estrogen priming	Secretory differentiation and preparation for implantation

Progesterone Transport in Blood

Because progesterone is hydrophobic, most circulating progesterone is associated with plasma proteins.

It binds principally to albumin and also to corticosteroid-binding globulin, while a relatively small fraction circulates unbound.

Metabolism of Progesterone

Progesterone is extensively metabolized, particularly in the liver. Its metabolites undergo additional modification and conjugation before elimination.

One important metabolite is pregnanediol, which can be excreted in conjugated form in urine.

Progesterone and Body Temperature

Progesterone has a thermogenic effect. Following ovulation, the progesterone-dominant luteal phase is associated with a small increase in basal body temperature.

This physiological change has historically been used as an indirect indicator that ovulation has occurred.

Basal Body Temperature

Basal body temperature typically increases after ovulation and remains relatively elevated during the luteal phase. If pregnancy does not occur, it generally falls as the corpus luteum regresses and progesterone declines.

Temperature patterns are indirect and can be influenced by many nonreproductive factors.

Progesterone and Respiration

Progesterone can stimulate respiratory drive. Increased progesterone during pregnancy contributes to changes in maternal ventilation.

This illustrates the systemic actions of progesterone beyond the reproductive organs.

Progesterone and the Central Nervous System

Progesterone and its metabolites can influence neural function. Progesterone receptors are expressed within the brain, and some progesterone metabolites interact with neurotransmitter systems.

These actions contribute to the broader neuroendocrine effects of reproductive steroid hormones.

Allopregnanolone

Allopregnanolone is a neuroactive metabolite of progesterone that can modulate GABA_A receptor activity.

It illustrates how metabolism of a steroid hormone can generate biologically active compounds with actions distinct from those mediated by classical nuclear progesterone receptors.

Progesterone in Males

Progesterone is also present in males, although circulating concentrations are generally lower than luteal-phase concentrations in females.

It serves as an intermediate in steroidogenesis and can exert biological effects through progesterone receptors in multiple tissues.

Progesterone and Adrenal Steroidogenesis

Within the adrenal cortex, progesterone-related intermediates participate in pathways leading to glucocorticoids, mineralocorticoids, and adrenal androgens.

The specific steroid products generated depend on the enzyme complement of each adrenal cortical zone.

Progesterone and Ovarian Steroidogenesis

Within the ovary, progesterone participates both as a secreted hormone and as a steroidogenic intermediate. The dominant steroid products change according to follicular stage, cell type, gonadotropin stimulation, and enzyme expression.

Progesterone and Inhibin

During the luteal phase, the corpus luteum produces both progesterone and inhibin A. These hormones contribute through different mechanisms to suppression of pituitary gonadotropin secretion.

Progesterone contributes broader hypothalamic-pituitary negative feedback, while inhibin A is particularly important for selective suppression of FSH.

Progesterone Deficiency

Inadequate progesterone exposure can impair normal secretory transformation of the endometrium and may occur when ovulation or corpus luteum function is disrupted.

Interpretation of progesterone status requires consideration of menstrual timing, ovulatory status, pregnancy status, and the clinical context.

Anovulation

In an anovulatory cycle, a normal postovulatory corpus luteum does not form. Consequently, the characteristic luteal rise in progesterone is absent.

The endometrium may therefore remain under predominantly estrogenic influence rather than undergoing normal progesterone-dependent secretory differentiation.

Unopposed Estrogen

When endometrial estrogenic stimulation occurs without adequate opposing progesterone, prolonged proliferative signaling can result.

This concept is important for understanding the relationship between chronic anovulation and abnormal endometrial proliferation.

Progesterone Measurement

Serum progesterone measurement can provide information about luteal activity when interpreted in relation to the timing of the ovarian cycle.

Because progesterone secretion can fluctuate substantially, a single measurement has limitations and must be interpreted in its clinical context.

Progesterone and Ovulation Assessment

A rise in progesterone after the expected time of ovulation provides biochemical evidence of luteal activity and is commonly used as an indirect indicator that ovulation has occurred.

The interpretation depends on appropriate timing because progesterone is normally low during the follicular phase.

Progesterone in Early Pregnancy

During early pregnancy, progesterone production by the corpus luteum is maintained by hCG. Progesterone supports the decidualized endometrium during the period before placental steroidogenesis becomes sufficient.

The relationship among hCG, the corpus luteum, progesterone, and the endometrium is therefore fundamental to early pregnancy endocrinology.

Progesterone Versus Estrogen

Feature	Progesterone	Estrogen
Hormone class	Steroid	Steroid
Major cyclic source	Corpus luteum	Developing ovarian follicle
Dominant cycle phase	Luteal phase	Follicular phase
Major endometrial action	Secretory differentiation	Proliferation
Cervical mucus	Generally thicker after ovulation	Thin and more permeable near ovulation
Breast effect	Promotes lobuloalveolar development	Promotes ductal development
Feedback	Predominantly negative feedback	Negative feedback and preovulatory positive feedback

Progesterone Versus Inhibin

Feature	Progesterone	Inhibin
Hormone type	Steroid	Glycoprotein
Important luteal source	Corpus luteum	Corpus luteum, particularly inhibin A
Major feedback action	Hypothalamic-pituitary negative feedback	Selective suppression of FSH
Major peripheral role	Endometrial differentiation and pregnancy support	Primarily reproductive endocrine regulation

Relationship to the Menstrual Cycle

Event	Progesterone Relationship
Follicular development	Progesterone remains relatively low
LH surge	Initiates luteinization leading to increased progesterone production
Ovulation	Followed by formation of progesterone-producing corpus luteum
Secretory phase	Driven largely by progesterone acting on estrogen-primed endometrium
No pregnancy	Corpus luteum regresses and progesterone falls
Menstruation	Triggered in part by withdrawal of progesterone and estrogen
Pregnancy	Progesterone production is maintained, initially through corpus luteum rescue by hCG

Relationship to Pregnancy

Stage	Major Progesterone Source or Function
Immediately after ovulation	Corpus luteum prepares endometrium for implantation
Early pregnancy	hCG maintains corpus luteum progesterone production
Developing pregnancy	Placental progesterone production increases
Established pregnancy	Placenta is the major progesterone source
Throughout pregnancy	Supports decidua and contributes to uterine quiescence and mammary development

Key Features of Progesterone

Feature	Key Point
Hormone type	Steroid hormone
Precursor	Cholesterol
Major cyclic source	Corpus luteum
Major pregnancy source	Placenta after early luteal support
Dominant ovarian-cycle phase	Luteal phase
Major uterine target	Endometrium and myometrium
Endometrial effect	Secretory differentiation and decidual support
Major receptors	PR-A and PR-B
Feedback	Predominantly negative within the HPG axis
Pregnancy role	Supports the endometrium and contributes to uterine quiescence
Breast effect	Promotes lobuloalveolar development
Temperature effect	Raises basal body temperature after ovulation

Anatomical and Physiological Importance

Progesterone provides the major endocrine link between ovulation and preparation of the uterus for pregnancy. The LH surge transforms the ruptured ovarian follicle into the corpus luteum, changing the ovary from a predominantly estrogen-producing follicular organ into a progesterone-dominant luteal endocrine organ.

The endometrium demonstrates the importance of sequential ovarian hormone action. Estrogen first stimulates proliferation during the follicular phase. Progesterone then acts on the estrogen-primed endometrium after ovulation, converting proliferative glands and stroma into a differentiated secretory tissue capable of supporting implantation. If pregnancy does not occur, regression of the corpus luteum removes progesterone support and contributes to menstruation.

If pregnancy occurs, trophoblastic hCG prevents the normal early regression of the corpus luteum. Continued luteal progesterone secretion maintains the endometrium until placental steroidogenesis becomes sufficiently established. The placenta then assumes the major role in progesterone production, supporting the decidua and contributing to a relatively quiescent uterus throughout gestation.

Progesterone also illustrates the integration of reproductive anatomy with endocrine feedback. Its production depends on structural transformation of the ovarian follicle, while its actions extend to the uterus, cervix, uterine tubes, breast, hypothalamus, pituitary, brain, and other tissues. Progesterone receptors allow these tissues to translate the circulating hormonal signal into tissue-specific responses.

Through these coordinated actions, progesterone regulates the transition from ovulation to the luteal phase, prepares the reproductive tract for implantation, supports early and established pregnancy, contributes to mammary differentiation, and helps control the cyclic activity of the hypothalamic-pituitary-ovarian axis.