Summary
Progesterone is the dominant hormone of the luteal phase — the second half of the menstrual cycle. Released by the corpus luteum after ovulation, it prepares the uterine lining for pregnancy and maintains early pregnancy. A correctly timed progesterone test (around day 21 of a 28-day cycle) is the standard way to confirm that ovulation has occurred.
After an egg is released at ovulation, the empty follicle becomes the corpus luteum, which secretes progesterone. This hormone transforms the uterine lining into a receptive environment for an embryo and, if pregnancy occurs, sustains it until the placenta takes over. If no pregnancy occurs, the corpus luteum breaks down, progesterone falls, and menstruation follows.
A ‘day 21’ progesterone (timed 7 days after ovulation, adjusted for cycle length) is the standard test to confirm ovulation in fertility assessment. Low progesterone in the luteal phase can indicate anovulation or luteal phase deficiency.
Progesterone also has calming, sleep-promoting effects via its metabolite allopregnanolone, and balances oestrogen’s proliferative effect on the uterine lining — which is why it is included in HRT for women with a uterus.
What It Is
Progesterone is a C21 steroid hormone synthesised from pregnenolone, itself derived from cholesterol. It is produced principally by the corpus luteum in the luteal phase, by the placenta during pregnancy, and in small amounts by the adrenal glands.
Progesterone acts via nuclear progesterone receptors to induce secretory transformation of the endometrium, suppress uterine contractility, and stimulate mammary development. Its neuroactive metabolite allopregnanolone is a positive allosteric modulator of GABA-A receptors, producing calming and sedative effects.
Reference ranges depend on cycle phase: follicular phase 30 nmol/L is reassuring, with 16–30 nmol/L borderline. Pregnancy levels are much higher. Postmenopausal < 2 nmol/L.
Functions
Ovulation confirmation
A rise in progesterone in the luteal phase is the standard biochemical proof that ovulation has occurred.
Pregnancy support
Prepares and maintains the uterine lining for implantation and sustains early pregnancy until the placenta takes over.
Mood and sleep
Its metabolite allopregnanolone has calming, sleep-promoting effects via GABA receptors; fluctuations contribute to PMS.
Endometrial protection
Balances oestrogen's proliferative effect, protecting the uterine lining — the reason progesterone is included in HRT for women with a uterus.
Reference Ranges
Progesterone (mid-luteal)
Measured in nmol/L| Status | Range (nmol/L) | Range (ng/mL) | What it means |
|---|---|---|---|
| Anovulatory / low | < 16 | < 5 | Low mid-luteal progesterone — suggests ovulation did not occur or luteal phase deficiency. |
| Borderline | 16–30 | 5–9.4 | Borderline — may indicate suboptimal ovulation; repeat or interpret with cycle timing. |
| Ovulatory | > 30 | > 9.4 | Confirms ovulation has occurred in a correctly timed mid-luteal sample. |
| Pregnancy range | > 70 | > 22 | High levels seen in pregnancy and supporting a viable corpus luteum. |
Interpretation depends entirely on cycle timing — the sample must be mid-luteal (7 days before next period). Ranges differ markedly in pregnancy. Always record cycle day and length.
Symptoms of Imbalance
Low progesterone disrupts the cycle and fertility; symptoms relate to luteal phase deficiency and oestrogen-progesterone imbalance.
- Irregular or short menstrual cycles
- Difficulty conceiving
- Spotting before periods
- Heavy or prolonged periods
- PMS, anxiety, and irritability
- Sleep disturbance
- Recurrent early miscarriage (luteal phase deficiency)
- Usually reflects pregnancy or progesterone therapy
- Drowsiness and fatigue
- Breast tenderness
- Bloating
- Mood changes
Causes of Imbalance
- Anovulation (no egg released)
- Luteal phase deficiency
- PCOS
- Perimenopause
- Hypothalamic dysfunction (stress, low body weight, excessive exercise)
- Hyperprolactinaemia
- Thyroid dysfunction
- Pregnancy
- Progesterone supplementation or therapy
- Congenital adrenal hyperplasia
- Ovarian cysts (corpus luteum cysts)
FAQs
It is a blood test timed to the mid-luteal phase — about 7 days after ovulation — to confirm that ovulation has occurred. In a textbook 28-day cycle this falls on day 21, but the timing must be adjusted for your actual cycle length: 7 days before your next expected period. A clearly elevated result confirms ovulation; a low result suggests ovulation did not occur or the test was mistimed.
Low mid-luteal progesterone usually means ovulation did not occur (anovulation) or was suboptimal (luteal phase deficiency). Common underlying causes include PCOS, perimenopause, thyroid problems, high prolactin, stress, and low body weight. Because mistiming the test also causes a falsely low result, the timing relative to your cycle is always checked first.
Progesterone maintains the uterine lining and suppresses uterine contractions, allowing an early pregnancy to implant and develop. In the first weeks it is produced by the corpus luteum, then by the placenta. Low progesterone in early pregnancy may be associated with miscarriage risk, and progesterone supplementation is sometimes used in women with recurrent early pregnancy loss.
Yes. Progesterone’s metabolite allopregnanolone acts on GABA receptors in the brain, producing calming and sleep-promoting effects. The natural fall in progesterone before menstruation contributes to premenstrual mood symptoms and sleep disturbance. This is also why some women notice mood and sleep changes when starting or stopping progesterone-containing treatments.
References
- Practice Committee of the American Society for Reproductive Medicine. Diagnostic evaluation of the infertile female. Fertil Steril. 2015;103(6):e44–e50. View source
- Csapo AI, et al. The significance of the human corpus luteum in pregnancy maintenance. Am J Obstet Gynecol. 1973;115(6):759–765. View source
- Schumacher M, et al. Progesterone synthesis and the nervous system. Front Neuroendocrinol. 2014;35(1):1–13. View source