• A normal menstrual cycle lasts 21 to 35 days, with bleeding typically lasting three to seven days.
• The menstrual cycle has distinct phases driven by hormones: menstruation, the follicular phase, ovulation, and the luteal phase.
• "Normal" varies from person to person, and slight variation cycle to cycle is completely healthy.

One of the most common period questions is also one of the trickiest: what's actually normal? A menstrual cycle is counted from the first day of one period to the first day of the next. A normal menstrual cycle can range from 21 to 35 days, with the average landing around 28 to 29 days. Bleeding itself usually lasts three to seven days. Your cycle is still considered regular even if it varies a little month to month.

Your cycle moves through distinct, hormone-driven stages:

Menstruation is your period, when the uterine lining sheds. Day one of bleeding marks the start of your cycle.

Beginning on day one and lasting until ovulation, this is when follicles in your ovaries mature an egg. The follicular phase is typically the longest phase of the cycle.

Around the midpoint (roughly day 14 in a 28-day cycle), a surge in luteinizing hormone triggers your ovary to release a mature egg.

After ovulation, your body prepares the uterine lining for a possible pregnancy. If no pregnancy occurs, hormone levels drop and your period begins, restarting the cycle.

Some menstrual cycle variation is healthy and normal, but certain patterns are worth noting, like cycles that are consistently shorter than 21 or longer than 35 days, periods lasting more than seven days, or flow that's dramatically heavier or lighter than usual. These don't always signal a problem, but they're worth mentioning at a checkup, especially if they're new.

Knowing your own baseline makes it much easier to notice when something shifts.

• A normal menstrual cycle ranges from 21 to 35 days, with bleeding lasting three to seven days, and slight variations from cycle to cycle are completely normal.
• The cycle moves through four hormone-driven phases — menstruation, the follicular phase, ovulation, and the luteal phase.
• Cycles consistently shorter than 21 days, longer than 35 days, or accompanied by unusually heavy or prolonged bleeding are worth mentioning to a provider, especially if they represent a change from your normal pattern.

Sources:

Menstrual Cycle (Normal Menstruation): Overview & Phases | Cleveland Clinic

The Normal Menstrual Cycle and the Control of Ovulation | NCBI Bookshelf