The menstrual phase: what happens and how it feels

The menstrual phase is the first phase of your cycle, the days when your body sheds the lining it built up in case of pregnancy. Day one of bleeding is also day one of your cycle, the marker every tracker uses to count forward from.

During this phase:

• The uterine lining breaks down and leaves the body as your period.
• Estrogen and progesterone are both at their lowest point of the month.
• Cramping often peaks in the first day or two, then eases.
• Flow is usually heaviest early on and tapers off by the last day or two.

Clinically, this phase is sometimes called menstruation or your period. Spotting a day before full flow starts still counts as day one once real bleeding begins, which is the convention most cycle trackers and clinicians use.

Most of the hormone shifts that drove PMS symptoms in the week before are already behind you by the time bleeding starts.

Bleeding marks the start of the menstrual phase and, by convention, the start of a new cycle. Length varies by person, and even cycle to cycle, so a single normal number doesn't exist.

These ranges come from NHS guidance on menstrual cycles, not one single study, so treat them as a general guide rather than a target. Stress, travel, some medications and conditions like PCOS can all shift the numbers around.

Day one matters for more than record-keeping. Apps and clinicians use it to estimate when ovulation and the next luteal phase are likely to land, usually by counting forward from it rather than guessing from symptoms alone.

Estrogen and progesterone both drop to their lowest levels right before bleeding starts, then stay low through most of the menstrual phase. That drop is what triggers the uterine lining to shed in the first place.

As the phase goes on, follicle-stimulating hormone (FSH) starts to rise, nudging the ovaries to prepare an egg for the next cycle. Cortisol, the main stress hormone, tends to run higher once you move into the follicular phase that follows than it did in the luteal phase before your period: a 2020 meta-analysis of 778 women found a modest but reliable difference (Hedges' g 0.13, p<0.01, Frontiers in Endocrinology).

That gradual shift from low to rising hormones is also why the first few days of bleeding can feel different from the last few, even though you're still in the same phase.

Pinpointing the exact day a hormone shift happens from symptoms alone is hard. A 2023 study of 96 women found common phase-dating methods often disagree with each other, with agreement no better than moderate (Hormones and Behavior, 2023). That's part of why trackers use bleeding, the one marker you can actually see, as day one.

None of this means your mood is only chemistry. It means the chemistry underneath it is shifting too.

Many people notice their mood steadies once bleeding starts, since the sharp hormone drop that often drives PMS has already happened. Energy can still be low for the first day or two, especially with cramping or heavy flow, before it lifts.

Common experiences:

• Relief or a flatter, calmer mood compared to the days before.
• Lower energy or a need for more rest early in the phase.
• Cramping, bloating or fatigue that eases as flow lightens.
• A clearer head for decisions that felt harder the week before.

Headaches, breast tenderness and bloating that built up before your period commonly ease alongside the mood lift, as the same hormone drop that eased PMS continues.

Across the whole cycle, mood doesn't stay flat. The menstrual cycle and mood guide walks through how it shifts in the other phases too, not just this one.

The menstrual phase is also a reset, a few days with some distance from the week that came before. For most people, PMS symptoms ease within a day or two of bleeding starting and don't return until the next luteal phase.

PMDD can behave differently. In a study of luteal-phase SSRI treatment, up to one third of women with PMDD still had some symptoms in the first two to three days after their period started, even though the treatment worked well overall (Journal of Affective Disorders, 2005). That's a reason to keep a few tracking notes into the start of bleeding, not just the days before it.

Three things worth doing in these first few days:

1. Write down how this month's hardest moment actually felt, before it fades.
2. Check a cycle phase calculator or hormone chart to see roughly when your next luteal phase starts.
3. If a fight with a partner is still on your mind, decide what you want to say about it while you have a clearer head.

Niyora is a PMS and PMDD support app. If that fight from last week still bothers you, Niyora's Moon lets you write down what happened, see it from another angle and draft a reply you can hold onto until you're ready, so it isn't lost by the time the next luteal phase starts.

The luteal phase page covers what to expect before your next period, and the menstrual cycle phases guide walks through all four stages together.