The menstrual cycle phase by phase: what your hormones do

One cycle, two halves and four hormones

The menstrual cycle is counted from the first day of bleeding and lasts between 24 and 35 days in most people, with month-to-month variation that is entirely normal. Over those days the body runs a carefully directed sequence with one goal: to mature an egg, release it, and prepare the uterus for a possible pregnancy.

Four hormones play the lead roles. FSH and LH (the pituitary's two signal hormones) are sent from the brain to the ovaries and direct the sequence. Estradiol (the body's main estrogen) and progesterone are made in the ovaries and do the actual work out in the body. Around them are others with influence, above all prolactin and the thyroid hormones, which can disturb the cycle when they sit wrong.

The cycle is usually divided into two halves, separated by ovulation: the follicular phase before and the luteal phase after. Understanding what the hormones do in each phase makes it easier to understand your own cycle, and, further on, why timing matters when hormones are measured.

The follicular phase: building up

The follicular phase begins at the same time as the period. The bleeding is the end of the previous cycle: when no pregnancy has occurred, hormone levels fall and the uterine lining is shed. Meanwhile the brain starts over. The pituitary increases its release of FSH, which wakes a group of egg follicles in the ovaries.

Over the following days the follicles grow, and one takes the lead. The growing follicle produces estradiol in rising amounts, and it is the estradiol that builds a fresh, nutrient-rich lining in the uterus. Many notice rising estradiol as more energy and better mood toward the end of the phase, although the experience varies a great deal from person to person.

The follicular phase is the variable part of the cycle. For one person it takes ten days, for another three weeks, and when cycles differ in length, this is usually where the difference sits. The luteal phase after ovulation is considerably more constant. Anyone using combined hormonal contraception usually has no ovulation at all; the method suppresses both FSH and LH, which is also why hormone tests taken during ongoing hormonal contraception are hard to interpret.

Ovulation: the LH surge

When the leading follicle is mature, estradiol reaches a peak, and that peak flips a switch in the brain. The pituitary answers with a strong, short-lived rise in LH, the so-called LH surge. Roughly a day to a day and a half later, the follicle ruptures and releases its egg, which is ovulation itself.

The egg then lives for about a day, while sperm can survive for several days in the uterus and fallopian tubes. The most fertile days are therefore the days before and during ovulation, not the days after.

The LH surge is also what urine ovulation tests measure. A positive test means the surge is under way and ovulation is likely coming, usually within a day or two. It is worth knowing that an LH surge is not proof that an egg was actually released; it is the progesterone of the next phase that can confirm that afterwards.

The luteal phase: progesterone's turn

After ovulation, the emptied follicle transforms into a corpus luteum, a small temporary hormone factory. It produces progesterone, the hormone that switches the lining from building to receiving, so that a fertilized egg can implant. Progesterone also raises body temperature by a few tenths of a degree, which is why temperature tracking can show afterwards that ovulation happened.

If there is no pregnancy, the corpus luteum lives for about two weeks. Then it gives up, progesterone and estradiol fall, the lining loses its support, and a new period begins. The luteal phase is strikingly constant, usually 11 to 17 days, however long the rest of the cycle is.

The days before the period, as hormones fall, are also the PMS days. Mood effects, tender breasts, bloating and disturbed sleep in that phase are common and hormonally understandable. When the symptoms are large enough to take over daily life, that is a reason to talk to a doctor, because help exists.

When the cycle goes off script: what hormone values can show

A cycle that turns irregular, stops or changes character is the body's way of signaling that something in the system has shifted. Sometimes the explanation is everyday: stress, weight loss, hard training and lack of sleep can all lean on the hormone system. Sometimes there is a measurable cause, and that is where a blood test can offer clues.

Raised prolactin can damp the signals from the pituitary and make the cycle sparse or absent. A thyroid that sits wrong, in either direction, can disturb the cycle in a similar way. In PCOS (polycystic ovary syndrome) there is often a pattern of raised testosterone or a shifted balance between LH and FSH. And as the ovaries approach menopause, FSH rises, while cycles often become irregular long before the final period.

A hormone test makes none of those diagnoses. What it does is show which track is worth following, and give you and a doctor something concrete to start from instead of guesses.

If you want to see where in the hormonal landscape you are, the cycle's baseline hormones, FSH, LH, estradiol and prolactin, can be measured in a single draw, with guidance on the right cycle day.

The right cycle day: how a hormone test is timed

Because the hormones move through the cycle, the same value means different things on different days. The timing is therefore part of the test.

The baseline hormones, meaning FSH, LH and estradiol, are usually measured on cycle day 2 to 5, counted from the first day of bleeding. The system is then at its starting point and the values are at their most comparable with the reference ranges. Prolactin can be measured at the same time; it is best taken in the morning and at rest, since stress and recently ended sleep can raise it temporarily.

Progesterone is the exception. It is measured about seven days before the expected period, in the middle of the luteal phase when the corpus luteum is at its most active, which in a 28-day cycle lands around day 21. A clearly raised progesterone there is a receipt that ovulation has taken place. With longer cycles the day moves later, and with a very irregular cycle the test can still be taken; the answer is then read with that in mind, and sometimes a test without exact timing is informative anyway, for example when the question is whether ovulation happens at all.

This article is general health information, not medical advice and not an assessment of your values. If you have symptoms or concerns, contact your healthcare provider.