The menstrual cycle is a repeating sequence of hormonal shifts that prepares your body for pregnancy each month, then resets if pregnancy doesn’t occur. A typical cycle lasts between 24 and 38 days, with 28 days as the average. The cycle has distinct phases, each driven by a precise chain of hormonal signals between your brain and ovaries.
The Hormonal Chain of Command
Everything starts in a small region of your brain that releases a signaling hormone (GnRH) in pulses, like a metronome. These pulses tell your pituitary gland, a pea-sized gland at the base of your brain, to release two key hormones: FSH (follicle-stimulating hormone) and LH (luteinizing hormone). FSH and LH travel through your bloodstream to your ovaries, where they trigger the growth of egg-containing follicles and the production of estrogen and progesterone.
What makes this system so precise is feedback. Your ovaries don’t just respond to brain signals; they send signals back. Rising estrogen from a maturing follicle suppresses FSH (so only one dominant follicle keeps growing) while simultaneously ramping up LH production. This back-and-forth communication is what drives each phase of the cycle forward and keeps the timing remarkably consistent.
The Follicular Phase: Building Up
The cycle begins on the first day of your period. During menstruation, which typically lasts three to seven days, the uterine lining from the previous cycle sheds. Total blood loss during a period is usually around 60 milliliters, roughly four tablespoons, though it often feels like more. Losing more than 80 milliliters regularly is considered heavy bleeding.
While you’re still bleeding, your brain is already setting the next cycle in motion. FSH rises during the early days, recruiting a small group of follicles in your ovaries to start developing. Over the next week or so, one follicle pulls ahead as the dominant follicle, and the rest stop growing. This dominant follicle produces increasing amounts of estrogen, which does two things at once: it thickens your uterine lining with a fresh blood supply, and it signals your pituitary to dial back FSH so no additional follicles compete.
The follicular phase is the most variable part of the cycle. In a 28-day cycle it lasts roughly 14 days, but it can be shorter or longer, which is the main reason cycle length differs from person to person and month to month.
The Estrogen Trigger and Ovulation
Ovulation hinges on a precise estrogen threshold. For the brain to flip from suppressing LH to flooding the body with it, estrogen levels must stay above 200 picograms per milliliter for roughly 50 hours. Once that threshold is crossed, estrogen’s effect reverses from negative feedback to positive feedback, and your pituitary releases a massive surge of LH.
This LH surge is the direct trigger for ovulation. It begins about 34 to 36 hours before the egg is actually released, which is why ovulation predictor kits that detect LH in urine can give you a day or more of advance notice. The follicle doesn’t rupture immediately. Research using direct observation found that no patients ovulated before 34 hours after the LH surge began, with about half ovulating between 37 and 39 hours after the surge started.
When the follicle does rupture, the egg is swept into the fallopian tube, where it remains viable for about 12 to 24 hours. Estrogen drops sharply right around this time, which is why some people notice a brief dip in energy or mood at mid-cycle.
Observable Signs of Fertility
Your body produces visible clues about where you are in the cycle, particularly through cervical mucus. In the days after your period, mucus is typically dry or sticky and paste-like, white or light yellow. As estrogen rises in the days before ovulation, it becomes creamy, then transitions to a clear, slippery, egg-white consistency. This wet, stretchy mucus is functional: its structure makes it easy for sperm to swim through the cervix and up toward the egg. After ovulation, mucus dries up or becomes thick again as progesterone takes over.
Body temperature also shifts. After ovulation, your resting (basal) temperature rises by about half a degree Fahrenheit, or 0.3 degrees Celsius. The rise is small enough that you need a sensitive thermometer and consistent morning measurements to catch it. Because the temperature shift happens after ovulation, it confirms that ovulation occurred rather than predicting it in advance.
The Luteal Phase: Holding or Resetting
After the egg is released, the empty follicle transforms into a temporary structure called the corpus luteum. This structure pumps out progesterone, the hormone that stabilizes and maintains the uterine lining. Progesterone also slows down the brain’s GnRH pulses to about one every four hours (compared to one every one to two hours earlier in the cycle), which prevents new follicles from developing while the body waits to see if the egg was fertilized.
If a fertilized egg implants in the uterine lining, it produces a hormone (hCG, the one pregnancy tests detect) that keeps the corpus luteum alive and producing progesterone for the first several weeks of pregnancy. If implantation doesn’t happen, the corpus luteum has a built-in expiration date of about 14 days, plus or minus two. It stops functioning three to four days before your period starts, though remnants of the structure linger into the next cycle. As progesterone drops, the thickened uterine lining loses its hormonal support and begins to break down, starting your period and resetting the cycle.
Unlike the follicular phase, the luteal phase is remarkably consistent. It almost always lasts 12 to 16 days regardless of overall cycle length. So if your cycles vary from month to month, the difference is nearly always in how long it takes to grow and ovulate a follicle, not in what happens afterward.
How Cycles Change With Age
Cycle length and regularity aren’t fixed throughout your reproductive years. In the first few years after periods begin, cycles are often irregular as the hormonal feedback system matures. Through your twenties and thirties, cycles tend to be the most predictable.
In the late reproductive years, most people notice little change, perhaps a slight increase in cycle variability. But as the pool of available follicles in the ovaries shrinks to a critical level, the early menopausal transition begins. Rising FSH levels cause follicles to develop faster, shortening the follicular phase and pulling cycles closer together. You might notice your cycle becoming a few days shorter than it used to be, or you might skip a period for the first time. Variation in cycle length exceeding seven days from your usual pattern is one hallmark of this stage.
In the late transition, cycles become highly irregular. Gaps of 60 days or more between periods are common, and ovulations can follow one another in rapid, unpredictable patterns with unusually short follicular phases. This stage ends when 12 consecutive months pass without a period, marking menopause. The entire transition from first irregularity to final period typically spans several years.