Published on:
March 23, 2026
Progesterone is a steroid hormone produced mainly by the ovaries. It plays a central role in the menstrual cycle, pregnancy, and broader aspects of health that many people never hear about. If you have ever had blood tests to check your fertility, experienced irregular periods, or discussed hormone replacement therapy (HRT) with a doctor, progesterone will have featured in the conversation. Despite its importance, it remains one of the less understood hormones compared to oestrogen or testosterone.
This guide explains where progesterone comes from, what it does throughout your life, and what happens when your levels are too low or too high.
Progesterone is classified as an endogenous steroid hormone. Your body produces it in several places, though the ovaries are the primary source in women of reproductive age. After ovulation each month, the empty follicle that released the egg transforms into a temporary gland called the corpus luteum. This structure pumps out progesterone during the second half of the menstrual cycle, known as the luteal phase.
The adrenal glands, which sit on top of each kidney, also produce smaller amounts of progesterone. Men produce progesterone too, in both the adrenal glands and the testes, where it serves as a precursor for testosterone and cortisol synthesis. During pregnancy, the placenta takes over progesterone production from roughly the tenth week onward, maintaining the high levels needed to sustain the pregnancy through to delivery.
Progesterone belongs to a broader class of hormones called progestogens. The term progestin more commonly refers to synthetic versions used in medications. The body also uses progesterone as a building block for other hormones, including cortisol and aldosterone. This means progesterone has effects well beyond the reproductive system.
The menstrual cycle has two main phases separated by ovulation. During the first half (the follicular phase), oestrogen dominates. It stimulates growth of the uterine lining and prepares a follicle in the ovary to release an egg. Progesterone levels remain low during this phase.
After ovulation, progesterone rises sharply. The corpus luteum secretes it in increasing amounts, and it transforms the uterine lining from a proliferative state into a secretory one. In practical terms, the lining becomes spongy, nutrient-rich, and receptive to a fertilised egg. Progesterone also thickens cervical mucus, making it harder for sperm to enter the uterus, and raises basal body temperature by about 0.3 to 0.5 degrees Celsius.
If no fertilised egg implants, the corpus luteum breaks down after roughly 12 to 14 days. Progesterone and oestrogen levels both fall, and this withdrawal triggers the shedding of the uterine lining: your period. The entire process then restarts. Women who experience severe premenstrual symptoms, including the more debilitating condition known as PMDD (premenstrual dysphoric disorder), may be particularly sensitive to these progesterone fluctuations during the luteal phase.
Tracking progesterone levels through a blood test on day 21 of a 28-day cycle is one of the most common ways doctors confirm whether ovulation has occurred. A level above 16 nmol/L is generally considered suggestive of ovulation, with levels above 30 nmol/L providing stronger confirmation.
When a fertilised egg implants in the uterine wall, it sends a signal (via human chorionic gonadotropin, or hCG) that tells the corpus luteum to keep producing progesterone instead of breaking down. This sustained progesterone output is essential in the first trimester for several reasons.
First, progesterone prevents the uterine muscles from contracting, which reduces the risk of the body expelling the embryo. Second, it supports the development of blood vessels in the uterine lining that will feed the growing placenta. Third, it modulates the immune system so the body does not reject the embryo as foreign tissue. Low progesterone levels in these early weeks are associated with a higher risk of miscarriage, though it can be difficult to determine whether low progesterone is a cause or a consequence of an already failing pregnancy.
Around weeks 8 to 10, the placenta matures enough to produce its own progesterone. This transition, sometimes called the luteo-placental shift, means the corpus luteum is no longer needed. Progesterone levels continue to climb throughout pregnancy, reaching concentrations far higher than anything seen during a normal menstrual cycle. By the third trimester, levels can be ten times higher than peak luteal phase values.
Some fertility clinics prescribe progesterone supplements during early pregnancy or after IVF to provide extra support during the period before the placenta takes over. These supplements come as vaginal pessaries, injections, or oral capsules. Your doctor will determine the most appropriate form based on your circumstances.
Low progesterone can result from several causes. The most straightforward is anovulation, where the ovary does not release an egg and no corpus luteum forms. Conditions such as polycystic ovary syndrome (PCOS), thyroid disorders, excessive exercise, significant stress, and very low body weight can all disrupt ovulation and therefore progesterone production.
Women with low progesterone may notice irregular or absent periods, spotting between periods, difficulty conceiving, and recurrent early miscarriage. Other symptoms can include mood changes, anxiety, difficulty sleeping, headaches, and breast tenderness. Because progesterone has a calming effect on the brain (it enhances the activity of GABA, a neurotransmitter that reduces neural excitability), low levels can leave you feeling more anxious or restless than usual.
When progesterone drops but oestrogen remains at normal or elevated levels, the imbalance is sometimes referred to as oestrogen dominance (a term used in clinical discussion, though it is not a formally defined medical diagnosis). This does not necessarily mean oestrogen itself is too high. Rather, the ratio between the two hormones has shifted. Symptoms associated with this imbalance can include heavier periods, worsening PMS, bloating, water retention, and fibrocystic breast changes.
A blood test is the most reliable way to measure progesterone. Your GP may request this alongside other hormone tests, including oestrogen, luteinising hormone (LH), follicle-stimulating hormone (FSH), and thyroid function, to get a full picture of your hormonal health.
As women approach their mid-to-late forties, ovulation becomes less frequent and less predictable. Cycles where no egg is released become more common, and each of these anovulatory cycles produces little to no progesterone. Oestrogen, by contrast, can fluctuate wildly during perimenopause, sometimes spiking to levels higher than those seen in younger women before dropping sharply.
This combination of erratic oestrogen and declining progesterone drives many perimenopausal symptoms: heavier periods, irregular cycle lengths, worsened mood swings, and disrupted sleep. After menopause (defined as 12 consecutive months without a period), both oestrogen and progesterone settle at consistently low levels.
For women who take oestrogen as part of HRT to manage menopausal symptoms, adding progesterone (or a synthetic progestogen) is necessary if the uterus is still intact. Oestrogen alone stimulates growth of the uterine lining. Without progesterone to oppose it, this unopposed oestrogen can lead to endometrial hyperplasia and, over time, increase the risk of endometrial cancer. The progesterone component of HRT counteracts this by regularly shedding or thinning the lining.
Micronised progesterone, a body-identical form derived from plant sources, has become the preferred option in many clinical guidelines. It is chemically identical to the progesterone your ovaries produce and tends to have fewer side effects than older synthetic progestogens. Women who have had a hysterectomy and no longer have a uterus do not typically need the progesterone component, though emerging research suggests it may offer additional benefits for sleep and mood.
Progesterone affects more than just the uterus and ovaries. Its influence extends to the brain, bones, cardiovascular system, and metabolism.
Progesterone and its metabolite allopregnanolone act on GABA receptors in the brain, producing a sedative and anxiolytic (anxiety-reducing) effect. This is one reason many women feel calmer during the mid-luteal phase when progesterone peaks, and more anxious or irritable just before their period when levels plummet. Research into allopregnanolone has led to new treatments for postnatal depression, highlighting how significant this pathway is for mental health.
While oestrogen gets most of the attention when it comes to bone density, progesterone may also contribute. Osteoblasts, the cells responsible for building new bone, have progesterone receptors. Some studies suggest that progesterone may stimulate bone formation rather than merely slowing bone breakdown (which is oestrogen's primary role). This complementary mechanism may help explain why women who frequently skip ovulation during perimenopause can lose bone density even before oestrogen levels fall significantly, though more research is needed in this area.
Progesterone has a mild relaxing effect on blood vessel walls and may help regulate blood pressure. It also influences how the body processes and stores fat. During the luteal phase, metabolic rate increases slightly, which is why some women notice greater hunger or increased caloric needs in the two weeks before their period. These effects are modest on their own but may become clinically relevant when progesterone levels are chronically low.
If your doctor suspects a progesterone-related issue, a blood test taken at the right time in your cycle is the standard approach. Timing matters: progesterone peaks around seven days after ovulation, so for a typical 28-day cycle, that means day 21. If your cycles are longer or shorter, your doctor will adjust the testing day accordingly.
Treatment depends on the underlying cause. For women trying to conceive, progesterone supplements during the luteal phase may improve the chances of successful implantation. For perimenopausal women, cyclical or continuous progesterone as part of HRT can stabilise bleeding patterns and reduce symptoms. For women with PCOS or other conditions causing anovulation, addressing the root condition through lifestyle changes, medication such as metformin, or ovulation-induction drugs may restore natural progesterone production.
Synthetic progestogens are also used in various hormonal contraceptives, including the mini-pill, hormonal IUDs (such as the Mirena coil), the injection, and the implant. These work partly by thickening cervical mucus, thinning the uterine lining, and in some formulations suppressing ovulation altogether. Your doctor can help you choose the right option based on your medical history and preferences.
Progesterone's main role is to prepare the uterine lining for a potential pregnancy each month and to maintain that lining during early pregnancy. Beyond reproduction, it influences mood, sleep quality, bone health, and metabolic rate. Both women and men produce progesterone, though women produce far larger amounts during their reproductive years.
Low progesterone relative to oestrogen can contribute to water retention and bloating, which may feel like weight gain. Over time, hormonal imbalance may also influence how the body stores fat, particularly around the hips, thighs, and abdomen. Addressing the underlying hormonal issue, through lifestyle modifications or medical treatment, can help.
Not exactly. Progesterone refers to the specific hormone your body produces naturally. Progestogen is a broader term that includes both natural progesterone and synthetic versions (progestins) used in medications like contraceptive pills and some forms of HRT. Micronised progesterone is considered body-identical because its chemical structure matches what the ovaries produce.
Your doctor may recommend a progesterone blood test if you have irregular periods, difficulty getting pregnant, recurrent miscarriages, or symptoms that suggest a hormonal imbalance. The test is most informative when taken about seven days after ovulation. Speak to your GP about the right timing based on your cycle length.
Progesterone and its metabolite allopregnanolone enhance the activity of GABA receptors in the brain, which promotes relaxation and sleepiness. Many women prescribed micronised progesterone as part of HRT report improved sleep quality. Taking it at bedtime, as is commonly advised, takes advantage of this natural sedative effect.
Disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. If you are experiencing severe or unusual symptoms, please seek prompt medical attention from your GP or an appropriate specialist.
The information provided in this article is for educational purposes only and is based on NHS recommendations. It is not a substitute for professional medical advice. Always consult your doctor or a qualified healthcare provider for advice on medical conditions or treatments.
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