What Is Progesterone and How Does It Affect Your Body?

Progesterone is a steroid hormone produced mainly by the ovaries after ovulation, with smaller amounts made by the adrenal glands and, during pregnancy, by the placenta. Its name literally means “pro-gestation,” reflecting its best-known role in preparing the uterus for pregnancy and sustaining it through the first trimester. But progesterone’s influence reaches far beyond reproduction: it shapes sleep, body temperature, immune function, skin quality, and even brain chemistry in ways that surprise most people.

Where Progesterone Comes From

Each month, after an egg is released from the ovary, the empty follicle transforms into a temporary gland called the corpus luteum. This structure churns out progesterone for roughly two weeks during the second half of the menstrual cycle, known as the luteal phase. If pregnancy occurs, the corpus luteum keeps producing progesterone until the placenta takes over around weeks 8 to 12. If no embryo implants, the corpus luteum breaks down, progesterone drops sharply, and menstruation begins.

The adrenal glands also produce small amounts of progesterone in both women and men, and the testes contribute some in men. Progesterone serves as a precursor molecule for other hormones too, including cortisol and aldosterone, so it has metabolic relevance well beyond the reproductive system. Once released into the bloodstream, progesterone acts through two receptor proteins, called A and B, that arise from a single gene and work by switching target genes on or off in different tissues.¹ The B form tends to be a stronger activator, while the A form can dampen or fine-tune that activity, and the balance between the two helps explain why progesterone does different things in different organs.²

Preparing the Uterus for Pregnancy

Progesterone’s most studied job is transforming the uterine lining after ovulation. During the first half of the cycle, estrogen drives the endometrium to thicken and proliferate. Once progesterone rises in the luteal phase, it shifts the endometrium from a proliferative state into a secretory one, producing nutrients and creating the right environment for an embryo to implant.³ This transformation includes structural changes to the glands and stroma of the endometrium, plus the secretion of specific proteins that support early embryo attachment.⁴

The timing matters. Progesterone can only do its work on endometrial tissue that has already been primed by estrogen during the proliferative phase. Without that estrogen exposure first, progesterone receptors are not adequately expressed, and the endometrium does not respond properly. This is why the two hormones work in sequence, not in competition. Peak progesterone during the mid-luteal phase corresponds to the window when the endometrium is most receptive to implantation, roughly six to ten days after ovulation.

What Happens When Progesterone Is Too Low

When the corpus luteum does not produce enough progesterone, or produces it for too short a time, the result is sometimes called luteal phase deficiency. The endometrium does not mature properly, which can make implantation difficult or increase the risk of early pregnancy loss.⁵ This condition is considered a plausible cause of infertility, though diagnosing it reliably is tricky because progesterone levels fluctuate throughout the day and from cycle to cycle.

Luteal phase deficiency can occur on its own or alongside other conditions. It is especially common during controlled ovarian stimulation used in fertility treatments, where the hormonal manipulation of ovulation can disrupt normal corpus luteum function.⁶ That is why progesterone supplementation, often given vaginally or by injection, is standard practice during IVF cycles. Outside of fertility treatments, low progesterone can contribute to irregular periods, spotting before your period actually starts, and difficulty maintaining early pregnancies.

Effects on the Brain, Sleep, and Mood

Progesterone does not just work in the uterus. Once in the body, it gets converted into a metabolite called allopregnanolone, which has potent effects on the brain. Allopregnanolone enhances the activity of GABA-A receptors, the same receptors targeted by anti-anxiety drugs and sleep aids.⁷ This is why many women feel sleepier or more relaxed during the luteal phase of their cycle, when progesterone is high. Research has confirmed that progesterone’s sleep-promoting effects closely resemble those of benzodiazepines and appear to be driven by the rise in allopregnanolone in the brain.⁸

The flip side is that the drop in progesterone and allopregnanolone just before menstruation appears to trigger mood symptoms in some women. In premenstrual dysphoric disorder (PMDD), the problem is not necessarily abnormal hormone levels but an abnormal brain response to normal fluctuations. The current evidence points toward a dysregulated sensitivity in GABA-A receptors to the changing levels of allopregnanolone across the cycle, which manifests as mood disturbance, anxiety, and difficulty managing stress responses.⁹ This distinction matters: PMDD is not caused by “too much” or “too little” progesterone, but by how the brain reacts to its natural rhythm.

Body Temperature and the Luteal Phase

If you have ever tracked your basal body temperature to monitor ovulation, you have already observed one of progesterone’s most reliable effects. After ovulation, the rise in progesterone pushes core body temperature up by roughly 0.3°C to 0.7°C compared to the first half of the cycle. This shift is most noticeable during sleep or right after waking, before any physical activity, which is why fertility awareness methods rely on morning temperature readings taken before getting out of bed. The temperature stays elevated throughout the luteal phase and drops back down when progesterone falls at the start of menstruation.

This thermoregulatory effect has practical consequences beyond fertility tracking. Some women notice they feel warmer, sleep hotter, or sweat more during the second half of their cycles. Athletes and researchers have also noted that the higher core temperature during the luteal phase can affect heat tolerance during exercise, a consideration that is increasingly recognized in sports science.

Immune System Modulation During Pregnancy

One of progesterone’s more remarkable roles emerges during pregnancy. A developing embryo carries genetic material from both parents, which means it is partly “foreign” from the mother’s immune system’s perspective. Progesterone helps solve this problem by broadly modulating the maternal immune system to promote tolerance of the embryo at the implantation site. It influences both the innate immune system (the first-responder cells) and the adaptive immune system (the more targeted, longer-term defenses) to prevent rejection while still allowing the mother to fight infections.¹⁰

This immunosuppressive shift is why pregnant women can be more susceptible to certain infections, including influenza and some viral illnesses. It is a trade-off: the immune system dials back its aggressiveness enough to tolerate the pregnancy, but in doing so it may leave the mother slightly more vulnerable to external threats.

Breast Tissue and Lactation

During pregnancy, progesterone contributes to the development of the milk-producing structures in the breast. Paradoxically, it also actively prevents milk production while the pregnancy is ongoing. Progesterone suppresses the enzyme needed to produce lactose, the sugar in breast milk, effectively keeping lactation on hold until after delivery.¹¹ When the placenta is delivered and progesterone levels plummet, that brake is released, and prolactin can finally trigger full milk production. This is why the dramatic hormonal shift after birth, not the act of breastfeeding itself, initiates the initial surge of milk.

Some women who retain placental fragments after delivery experience delayed milk production, and one suspected reason is that leftover tissue continues to produce progesterone, maintaining the suppressive signal. The timing of milk “coming in” aligns closely with the fall in progesterone during the first few days postpartum.

Skin, Elasticity, and Sebum

Progesterone affects skin in several ways, some welcome and some less so. Together with estrogen, it helps maintain skin thickness by stimulating the growth of skin cells and reducing the breakdown of collagen.¹² A randomized trial of 2% progesterone cream applied to peri- and postmenopausal women found measurable improvements in skin elasticity, reductions in wrinkle depth, and increased skin firmness compared to placebo, with the treatment group showing roughly a 24% gain in firmness versus about 13% with the vehicle cream alone.¹³

On the less welcome side, progesterone stimulates sebum production. This is one reason why breakouts tend to cluster in the days before menstruation, when progesterone has been high throughout the luteal phase while estrogen has already started to decline. The relative dominance of progesterone at that point translates into oilier skin and clogged pores for some people. If you have noticed a predictable acne pattern tied to your cycle, progesterone-driven sebum output is the likely explanation.

Progesterone in Men

Progesterone is often treated as exclusively a female hormone, but men produce it too, primarily in the adrenal glands and testes. In men, progesterone plays a role in sperm maturation, the acrosome reaction (the process that lets a sperm penetrate an egg), and the production of testosterone in the Leydig cells of the testes.¹⁴ Progesterone receptor activity in the prostate has also attracted attention as a potential marker in prostate conditions, though this area of research is still developing. Men’s progesterone levels are lower than those seen in women during the luteal phase, but the hormone is far from irrelevant to male physiology.

How Progesterone Is Used in Contraception

Synthetic versions of progesterone, called progestins, are the backbone of many contraceptive methods. Progestin-only pills, hormonal IUDs, implants, and injections all leverage progesterone-like activity to prevent pregnancy through several overlapping mechanisms. The most important of these are suppressing or disrupting the hormonal surge that triggers ovulation and thickening cervical mucus to block sperm from reaching the egg.¹⁵ Some progestin-based methods also alter the endometrial lining, though how much this contributes to contraceptive effectiveness remains debated.¹⁶

An important distinction here is between natural progesterone and synthetic progestins. While both activate progesterone receptors, synthetic progestins can also bind to other steroid receptors, including androgen, glucocorticoid, and mineralocorticoid receptors, which gives them different side-effect profiles. Some progestins cause acne or mood changes that natural progesterone does not, and vice versa. A class effect cannot be assumed between natural progesterone and synthetic progestins in terms of either efficacy or safety.¹⁷

Progesterone in Menopause and Hormone Therapy

After menopause, the ovaries stop ovulating, and progesterone production drops to very low levels. For women who take estrogen therapy to manage hot flashes and other menopausal symptoms, adding a progestogen is essential if they still have a uterus. Without it, unopposed estrogen can cause the uterine lining to thicken abnormally, raising the risk of endometrial hyperplasia and cancer. A large systematic review of 84 randomized trials found that most progestogen regimens studied provided effective endometrial protection for the doses and durations examined.¹⁸

The REPLENISH trial, which tested a combined oral capsule of estradiol and progesterone at several dose levels, reported no cases of endometrial hyperplasia or cancer over 12 months of continuous use, while the two highest doses significantly reduced the frequency and severity of hot flashes.¹⁹ Topical progesterone creams have also shown some antiproliferative effects on the endometrium, though circulating blood levels tend to be lower with transdermal delivery, and the evidence for full endometrial protection through creams alone is less established than for oral or vaginal routes.²⁰

An Ancient Hormone With an Unusual Evolutionary Twist

Progesterone is not unique to mammals. Versions of the progesterone receptor exist across vertebrates, from fish to birds to reptiles. But the story has an interesting wrinkle. In land-dwelling vertebrates, including humans, the progesterone receptor responds strongly to progesterone itself. In ray-finned fish like zebrafish, however, the receptor evolved to respond primarily to a different steroid, 17,20β-dihydroxy-progesterone, and responds only weakly to progesterone. Both lineages inherited their receptor from a common ancestor whose receptor recognized both steroids, but evolution pushed each group toward a different preferred ligand.²¹ This divergence matters for environmental science, because chemicals that disrupt the progesterone receptor in fish may not have the same effect in humans, and vice versa. Researchers studying endocrine disruptors need to account for this split when extrapolating across species.

Progesterone was first isolated and structurally characterized in 1934 by several independent research groups, a milestone that opened the door to understanding menstrual cycle regulation and ultimately to the development of oral contraceptives decades later. The hormone’s discovery also drove research into how steroid hormones work at the molecular level, influencing endocrinology as a whole. It is a molecule with a longer research pedigree than most people realize, and scientists are still uncovering new aspects of its activity in tissue repair, neuroprotection, and metabolic regulation.