The four menstrual cycle phases explained

A menstrual cycle typically lasts between 21 and 35 days, measured from the first day of one period to the first day of the next. The average is around 28 days, but this is just an average — significant variation is normal. Cycle length also changes across the lifespan, often shortening in perimenopause.

Not necessarily. Day 14 is the average ovulation day for a 28-day cycle, but ovulation timing varies considerably. If your cycle is 35 days, you might ovulate around Day 21. If it's 24 days, you might ovulate around Day 10. Cync estimates ovulation based on your logged cycle history but always labels these as estimates — not certainties.

PMS (premenstrual syndrome) is associated with the hormonal changes in the luteal phase — particularly the fall in estrogen and progesterone before menstruation. Lower estrogen affects serotonin levels, which influences mood. The exact cause varies between individuals. Stress, poor sleep, and nutritional factors can worsen PMS symptoms.

Hormonal fluctuations throughout the cycle influence the brain, nervous system, metabolism, and immune function. Estrogen supports serotonin and dopamine; progesterone has a calming (sometimes sedating) effect; falling hormones before menstruation can increase emotional sensitivity. These effects are real — but they vary significantly between individuals.

Yes. The stress hormone cortisol can disrupt the hypothalamic-pituitary-ovarian axis — the hormone communication pathway that regulates the cycle. Chronic or acute stress can delay or suppress ovulation, shorten or lengthen the luteal phase, and sometimes cause missed periods. This is a normal protective response, not a sign that something is permanently wrong.