- 01Cycle length changes before ovulation, not after
The follicular phase stretches from about 10 to 30 days; the luteal phase averages about 12 to 14 and varies far less. So ovulation, and the mid-luteal progesterone check, are dated by counting back from the next period, and a hormone result only means something next to the cycle day it was drawn on.
Across one cycle, the brain and ovary run a loop. The pituitary sends FSH to grow follicles (small fluid-filled sacs in the ovary, each holding one egg), the follicles send back estradiol, a spike of LH releases the egg, and the empty follicle turns into a short-lived gland called the corpus luteum, which makes progesterone until the next period. Cycle day 1 is the first day of full bleeding, not spotting. Every hormone level on a lab report is a snapshot of one point in that loop.
The control loop: brain, pituitary, ovary
The cycle runs on feedback. The hypothalamus, a small control centre at the base of the brain, releases GnRH (gonadotropin-releasing hormone) in pulses. GnRH tells the pituitary, the gland just below it, to release FSH (follicle-stimulating hormone) and LH (luteinising hormone). Those two tell the ovary what to do, and the ovary reports back through its own hormones. Estradiol does both jobs in this loop: it first brakes FSH and LH, then, once it stays high, triggers the LH surge 3.
How the brain-to-ovary loop produces one ovulation
What each hormone does, and when it peaks
| Hormone | Made by | Main job | Highest | Lowest |
|---|---|---|---|---|
| FSH | Pituitary | Recruits and grows follicles | Late in the previous cycle into days 1 to 5, plus a small spike with the LH surge | Mid-luteal phase |
| LH | Pituitary | Triggers ovulation, then supports the corpus luteum | The surge, about 1 to 1.5 days before ovulation | Luteal phase |
| Estradiol (E2) | Growing follicle, then corpus luteum | Thickens the uterine lining, flips the LH switch | About a day before ovulation, with a smaller second rise mid-luteal | During the period |
| Progesterone | Corpus luteum | Readies the lining for implantation, raises body temperature | About 7 days after ovulation | Follicular phase |
| Inhibin B and inhibin A | Follicles (B), corpus luteum (A) | Fine-tune FSH | B in the follicular phase, A mid-luteal | B after ovulation, A in the follicular phase |
| Testosterone | Ovary and adrenal glands | Precursor for estradiol; libido | A modest rise around mid-cycle | Early follicular phase |
| AMH | Small follicles | Reflects how many small follicles are growing | Fairly flat across the cycle | Fairly flat across the cycle |
Units differ by lab. Estradiol comes back in pg/mL in most US labs and pmol/L elsewhere; progesterone in ng/mL or nmol/L. Compare a result only with the range printed on the same report. AMH barely moves with the cycle, so an AMH test can be drawn on any day, while FSH and estradiol cannot.
The cycle hormones you can test
Days 1 to 5: your period
During the period, estradiol and progesterone sit at their lowest. FSH has already started climbing in the last days of the previous cycle, as the old corpus luteum faded and stopped holding it down. That early FSH rise recruits the next group of follicles. Days 2 to 5 are when labs compare FSH, LH and estradiol with baseline ranges, because the hormones are at their quietest and most comparable between people 3.
Follicular phase: estradiol builds
FSH recruits several follicles, and one becomes dominant. The dominant follicle pours out estradiol, which rebuilds the uterine lining, and inhibin B, which pulls FSH back down so the other follicles stall 3. The follicular phase is the stretchy part of the cycle. In the Bull 2019 dataset of 612,613 app-tracked ovulatory cycles, the mean follicular phase was 16.9 days, with 95% of cycles falling between 10 and 30 days 1.
Ovulation: the LH surge
Estradiol peaks about a day before ovulation. The LH surge begins roughly 34 to 36 hours before the egg is released and peaks 10 to 12 hours before, with a smaller FSH spike alongside 3. Urine ovulation tests detect the LH surge, so a positive test means ovulation is likely within about a day and a half. They can also read positive without ovulation in PCOS and perimenopause, when LH runs high. Day-counting and ovulation tests help with timing, but on their own they are not reliable contraception; a clinician can go through methods that are.
Among healthy women trying to conceive, nearly all pregnancies can be attributed to intercourse during a six-day period ending on the day of ovulation.Wilcox, Weinberg and Baird, New England Journal of Medicine, 1995 2
That six-day fertile window is the five days before ovulation plus ovulation day itself, because sperm can survive for several days while the egg lasts about a day 2. Other signs of ovulation include clear, stretchy cervical mucus in the days before, mild one-sided pelvic pain for some people, and a small rise in resting temperature that shows up only after ovulation has passed. Tracking these signs, or using urine LH tests, is how an ovulation tracker estimates the day.
CalculatorOvulation calculatorEnter your usual cycle length to see where your fertile window probably falls. It is an estimate from averages, not a method of birth control.Work it out Trying to conceive? The fertility guide covers which tests to ask for and when.
Luteal phase: progesterone takes over
The corpus luteum makes progesterone, a second, smaller wave of estradiol, and inhibin A. Progesterone raises resting body temperature by a few tenths of a degree Celsius and peaks about 7 days after ovulation 3. The luteal phase is the steadier half of the cycle: in Bull 2019 the mean was 12.4 days, with 95% of cycles between 7 and 17 days 1.
If an embryo implants, it makes hCG, which keeps the corpus luteum alive and progesterone high (hCG is the hormone a pregnancy test detects). If no embryo implants, the corpus luteum breaks down after about two weeks, estradiol and progesterone fall together, the lining sheds, and FSH starts rising for the next round.
Because the luteal phase is steadier, you can estimate when ovulation happened by counting back about 12 to 14 days from the first day of the next period. Counting forward 14 days from day 1 only works if the follicular phase happens to be 14 days long, and in real cycles it often is not.
Why you can feel different week to week
Premenstrual symptoms are tied to hormone change, but not to abnormal hormone levels. In a 1998 study, Schmidt and colleagues switched off the cycle with a drug, and symptoms went away in women with PMS. When estradiol or progesterone was added back, symptoms returned in the women with PMS and not in the women without it, even though the hormone levels were the same 6.
In women with a history of premenstrual syndrome, the symptoms appear to result from an abnormal response to normal hormonal changes.
The late-luteal drop in estradiol and progesterone is the common trigger. Energy, appetite and mood do not shift the same way for everyone in each phase, so a phase-by-phase plan will fit some people and not others. If mood shifts are large or hard to live with, bring them up with a doctor, since severe premenstrual symptoms (PMDD) are recognised and treatable; the mood swings page covers what to look at.
What normal looks like in real cycles
FIGO, the international federation of obstetricians and gynaecologists, defines a normal adult cycle as 24 to 38 days, with cycle-to-cycle variation of up to 7 to 9 days depending on age, and bleeding that lasts up to 8 days 4. In real tracked cycles, only about 13% were exactly 28 days, and the mean was 29.3 days 1.
Cycle length also shifts with age. Cycles shortened by about 0.18 days per year between ages 25 and 45, almost all of it in the follicular phase 1. Periods usually start around age 12, and cycles are often irregular for the first few years. In the 40s, perimenopause brings the next spell of irregularity: cycles that differ by 7 days or more, again and again, are an early sign of the menopause transition. As our perimenopause page puts it, "FSH and estradiol swing from week to week, so a normal FSH such as 5.4 mIU/mL does not rule it out." Periods stop for good at menopause, at about 51 on average.
Where an example cycle length fallsdays
What this means for a hormone blood test
The same estradiol value can be normal on day 12 and odd on day 3, so most hormone results need the cycle day beside them. Two windows cover most tests. FSH, LH and estradiol are read early in the follicular phase, days 2 to 5. Progesterone is read in the mid-luteal phase, about 7 days after ovulation or 7 days before the expected period, to confirm that ovulation happened 5.
The familiar day 21 progesterone test assumes a 28-day cycle. NICE guidance says to measure progesterone mid-luteal and, for longer or irregular cycles, to test later and repeat weekly until the next period 5. For a 35-day cycle, the mid-luteal point is around day 28, so a day 21 progesterone can look low only because ovulation has not happened yet. A low progesterone result taken on the wrong day may be a timing problem rather than a hormone problem, so ask the clinician who ordered it whether to repeat it at the right point in your cycle.
If you have a result in front of you, work out which cycle day it was drawn on (day 1 is the first day of full bleeding). If FSH, LH or estradiol was not drawn on days 2 to 5, or progesterone was not drawn about 7 days before your next period, ask your clinician whether to repeat it on the right day before reading anything into it.
| Pattern on the report | When drawn | What it can point to |
|---|---|---|
| High androgens such as testosterone, with irregular cycles | Any day, ideally early follicular | A feature of PCOS; as that page notes, "No single blood test shows PCOS" |
| High FSH with low estradiol | Days 2 to 5 | Fewer follicles left, as in perimenopause; under age 40 this can signal primary ovarian insufficiency and needs a clinician's review |
| Low progesterone | Mid-luteal, timed to your cycle | No ovulation that cycle |
| High prolactin | Any day, ideally morning | A common cause of missed or irregular periods |
For the full panel, test by test, see the female hormone panel explained. One result is a single point on a moving curve, which is why repeat readings beat single ones here more than almost anywhere else.
Also check when cycles are irregular
When the rhythm changes
Irregular periods usually mean the curve has shifted. Combined hormonal contraception works mainly by suppressing the LH surge, so the natural curve flattens and cycle-phase reference ranges do not apply while taking it. The bleed on a pill break is a withdrawal bleed, not a period that follows ovulation. Pregnancy and breastfeeding also hold ovulation off. Outside those, the common causes are low energy availability (eating too little for the training load), heavy stress, thyroid disease (underactive or overactive), high prolactin, PCOS and perimenopause. A late or absent period has its own page: missed period.
Citations
- Bull JR, Rowland SP, Scherwitzl EB, et al. Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine 2019;2:83.
- Wilcox AJ, Weinberg CR, Baird DD. Timing of sexual intercourse in relation to ovulation: effects on the probability of conception, survival of the pregnancy, and sex of the baby. N Engl J Med 1995;333:1517-1521.
- Reed BG, Carr BR. The Normal Menstrual Cycle and the Control of Ovulation. Endotext, updated 2018.
- Munro MG, Critchley HOD, Fraser IS; FIGO Menstrual Disorders Committee. The two FIGO systems for normal and abnormal uterine bleeding symptoms and classification of causes of abnormal uterine bleeding in the reproductive years: 2018 revisions. Int J Gynaecol Obstet 2018;143:393-408.
- National Institute for Health and Care Excellence. Fertility problems: assessment and treatment (CG156). 2013, updated 2017.
- Schmidt PJ, Nieman LK, Danaceau MA, Adams LF, Rubinow DR. Differential behavioral effects of gonadal steroids in women with and in those without premenstrual syndrome. N Engl J Med 1998;338:209-216.
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
