What is the menstrual cycle?

The menstrual cycle is the hormonal cycle that runs from the first day of one period to the first day of the next, usually 21 to 35 days long. It's driven by estrogen and progesterone rising and falling together, levels worth tracking directly rather than guessed from a calendar date (Psychoneuroendocrinology, 2021).

The four phases, in order:

• Menstrual: your period, with both hormones low
• Follicular: estrogen rises as an egg matures
• Ovulation: estrogen peaks, then drops as an egg releases
• Luteal: progesterone rises, then both hormones fall before the next period

Cycle length and the exact timing of each phase vary from person to person. The menstrual cycle hormones, phase by phase page breaks down what estrogen and progesterone are doing day by day.

PMS and PMDD both show up in the luteal phase, the one to two weeks before your period when progesterone rises and then both hormones fall.

People don't respond to that drop the same way. The premenstrual and menstrual days carry measurably higher risk for some:

• Psychiatric admissions run about 13% higher in the premenstrual phase (Archives of Suicide Research, 2019).
• Suicide attempts run about 17% higher around the time of menstruation (Archives of Suicide Research, 2019).
• People with an existing psychotic disorder see admission rates around 1.5 times higher in the days around their period (Schizophrenia Bulletin, 2020).

PMS brings physical or emotional symptoms that ease once bleeding starts, and it's common. PMDD is a diagnosis in the DSM-5-TR: at least five symptoms, including one mood symptom, confirmed by daily ratings over two cycles. Read more on PMDD (premenstrual dysphoric disorder), explained.

Estrogen and progesterone shifts can reshape how your brain responds to mood changes day to day. Neuroimaging research across 1,304 naturally cycling women found that estrogen and progesterone changes affect the structure and reactivity of brain regions involved in emotion, especially cortico-limbic areas (Frontiers in Neuroendocrinology, 2021).

Cortisol, the body's main stress hormone, runs higher in the follicular phase than the luteal phase, a pattern researchers link to neurosteroids moving in the opposite direction (Frontiers in Endocrinology, 2020). Much of this evidence is newer than it sounds.

A 2021 methods guideline found that decades of menstrual cycle research used inconsistent definitions of 'phase', which limited what scientists could compare (Psychoneuroendocrinology, 2021). Newer studies increasingly measure hormones directly instead of guessing phase from calendar day.

A few related terms come up alongside 'menstrual cycle' in research and everyday tracking:

• Luteal phase: the stretch after ovulation when PMS and PMDD symptoms build. See the luteal phase: what's happening in your body and mood.
• Corpus luteum: the structure that forms after ovulation and makes progesterone. See Corpus Luteum: what it means.
• Allopregnanolone: a hormone byproduct tied to mood changes in PMDD. See Allopregnanolone: what it means.

For a broader look at the evidence behind all of this, the PMS and PMDD research, summarised page rounds up the key studies. Niyora is a PMS and PMDD support app that gives you an estimated PMS window and a place to write down what you're feeling before deciding what to do with it.