Hormones and Your Menstrual Cycle: FSH, Estrogen, LH, and Progesterone Explained
The four hormones that control your menstrual cycle — what each one does, when it's tested, and what abnormal results mean for fertility.
<!-- H2 not in keyword doc — used card title as fallback: What is AMH and what does it measure? — AMH is not covered in this source deck; see ovarian-reserve deck --> Four hormones run your fertility. Understanding what they do at each phase of the cycle changes how you read your body, your lab results, and your fertility workup.
What hormones control the menstrual cycle?
Four hormones coordinate the cycle: FSH (follicle-stimulating hormone) recruits follicles at the start of the cycle. Estrogen builds the uterine lining and triggers the LH surge. LH (luteinizing hormone) triggers ovulation 24–36 hours after its surge. Progesterone sustains the uterine lining after ovulation to support implantation. When fertility testing is ordered, these hormones are what's being measured — each one has a specific, testable job.
What is FSH and what does a high FSH level mean?
FSH recruits follicles at the start of each cycle. Day 3 FSH is a standard fertility test — an elevated level can signal that the ovaries are working harder than expected to produce a follicle, which can reflect diminished ovarian reserve. It's checked alongside estradiol, because high estradiol on day 3 can suppress FSH and make it look falsely normal — the two values are interpreted together, not in isolation.
What does estrogen do during your cycle?
Estrogen — specifically estradiol — builds the uterine lining during the follicular phase and is what makes cervical mucus change from sticky to clear and stretchy as ovulation approaches. The rising estradiol level also triggers the LH surge, which then triggers ovulation. Day 3 estradiol is checked with FSH because an elevated estradiol can mask an otherwise elevated FSH, making it appear falsely normal.
What is LH and why does it surge before ovulation?
LH surges 24–36 hours before ovulation — this is the signal that causes the mature follicle to release the egg. It's exactly what ovulation predictor kits (OPKs) detect. In PCOS, baseline LH is often chronically elevated, which can make OPKs unreliable: multiple apparent "positives" that don't reflect an actual impending ovulation. If your LH appears elevated all the time, that's a conversation for your provider.
What is progesterone and why does it matter for fertility?
After ovulation, the follicle becomes the corpus luteum and begins producing progesterone. Progesterone sustains the uterine lining for implantation and supports the early pregnancy. Low mid-luteal progesterone can indicate that ovulation didn't occur, or that the luteal phase isn't being adequately maintained. It's also why PMS and early pregnancy symptoms feel identical — progesterone drives most of both.