Progesterone is used to support pregnancy, regulate menstrual cycles, and protect the uterine lining during hormone replacement therapy. It’s one of the two main female sex hormones (alongside estrogen), and its medical applications span fertility treatments, menopause management, and correction of absent or irregular periods. Whether your body isn’t making enough on its own or you need supplemental support during a specific treatment, progesterone fills several distinct roles.
What Progesterone Does in the Body
Progesterone’s primary job is preparing the uterus for pregnancy each month. After ovulation, the structure left behind by the released egg (called the corpus luteum) starts producing significant amounts of progesterone. This triggers a chain of changes: the uterine lining thickens and develops a rich network of blood vessels, the glands within the lining begin coiling and secreting nutrients, and cervical mucus thickens. All of these changes create a hospitable environment for a fertilized egg to implant and receive nourishment.
Progesterone also raises your body temperature slightly after ovulation, which is why tracking basal body temperature can help identify when you’ve ovulated. If pregnancy doesn’t occur, progesterone levels drop sharply, and the built-up uterine lining sheds as a period. This rise-and-fall cycle is what keeps menstruation regular.
Protecting the Uterus During Hormone Therapy
For women going through menopause who still have a uterus, progesterone is a critical addition to estrogen therapy. Taking estrogen alone causes the uterine lining to thicken continuously, a condition called endometrial hyperplasia that can become a precursor to uterine cancer. Adding progesterone counteracts this by thinning the lining on a regular schedule, mimicking the natural cycle.
The protection is substantial. Combined estrogen-progesterone therapy doesn’t just neutralize the cancer risk from unopposed estrogen. It actually reduces endometrial cancer risk by about 35% compared to taking no hormones at all, according to National Cancer Institute data. This is why any woman with an intact uterus who takes estrogen for hot flashes, sleep disruption, or other menopause symptoms will typically be prescribed progesterone alongside it.
Restarting Absent Periods
When menstrual periods stop for reasons other than pregnancy or menopause (a condition called secondary amenorrhea), progesterone can be used to trigger a withdrawal bleed. The concept is straightforward: a short course of progesterone mimics the hormonal signal your body normally sends at the end of a cycle. When you stop taking it, your progesterone levels drop, and the uterine lining sheds just as it would in a natural period.
This approach also serves as a diagnostic tool. If a withdrawal bleed occurs after progesterone, it confirms that your body is producing enough estrogen to build up the uterine lining and that the physical pathway for menstruation is intact. The bleeding typically starts two to seven days after completing the progesterone course. A standard oral protocol uses 400 mg taken at bedtime for 10 days, though vaginal formulations are also available.
Supporting Pregnancy and Preventing Preterm Birth
Progesterone plays a role in fertility treatment and early pregnancy support. Because low progesterone during the second half of the menstrual cycle (a luteal phase defect) is one suspected cause of miscarriage, supplementation is sometimes prescribed for women with recurrent pregnancy losses. The logic is that boosting progesterone helps maintain the uterine lining long enough for an embryo to implant securely. Women undergoing IVF routinely receive progesterone support because the egg retrieval process disrupts the corpus luteum that would normally produce it.
For preterm birth prevention, the picture has shifted in recent years. The FDA withdrew approval of the injectable synthetic form (17-alpha hydroxyprogesterone caproate) in 2023 after updated evidence showed it wasn’t effective. Current guidance from the American College of Obstetricians and Gynecologists is more targeted: vaginal progesterone may help prevent preterm birth in women who have both a history of preterm delivery and a shortened cervix. Without a shortened cervix, progesterone is no longer recommended for preterm birth prevention, even with a prior preterm delivery.
How Progesterone Is Taken
Progesterone comes in oral capsules, vaginal inserts or gels, and injectable forms, and the route matters more than you might expect. Oral progesterone has limited absorption because the hormone doesn’t dissolve easily in the digestive system. It’s typically taken at bedtime because it can cause drowsiness, a side effect related to how it’s metabolized in the liver. For endometrial protection during hormone therapy, the standard oral dose is 200 mg nightly for 12 days of each 28-day cycle.
Vaginal progesterone bypasses the liver and delivers the hormone more directly to the uterus, which makes it preferred for fertility support and preterm birth prevention. Topical creams exist but lack reliable evidence for most medical uses. In particular, topical progesterone doesn’t reach sufficient levels in either the blood or uterine tissue to protect the endometrium, so it should not be used as a substitute for oral or vaginal forms during estrogen therapy.
Bioidentical vs. Synthetic Progestins
Not all progesterone prescriptions contain the same molecule. Bioidentical progesterone, often called micronized progesterone, is derived from plant sources and has the exact same molecular structure as the progesterone your body makes. Synthetic progestins are lab-created molecules that activate many of the same receptors but have a different chemical structure.
This distinction matters clinically. The Women’s Health Initiative study in the early 2000s found that adding a synthetic progestin to estrogen therapy increased breast cancer risk. Later research suggested that bioidentical progesterone does not carry the same elevated risk and may even offer some protective effect. Both types effectively protect the endometrium, but the differing safety profiles for breast tissue have made bioidentical progesterone the preferred option for many prescribers, particularly in menopause hormone therapy.
Common Side Effects
Oral progesterone frequently causes drowsiness, which is why it’s dosed at bedtime. Some women also experience bloating, breast tenderness, headaches, or mood changes. These side effects tend to mirror what many women feel in the second half of their natural menstrual cycle, since that’s when progesterone levels are naturally highest. Vaginal progesterone causes fewer systemic side effects like drowsiness but can cause local irritation. Most side effects are dose-dependent and resolve after stopping the medication or adjusting the form.