---
title: "Menses Phase | Anatomy and Physiology I"
description: "Menses phase is the start of the menstrual cycle, when the endometrium sheds and menstrual bleeding begins in Anatomy and Physiology I."
canonical: "https://fiveable.me/anatomy-physiology/key-terms/menses-phase"
type: "key-term"
subject: "Anatomy and Physiology I"
unit: "Unit 27"
---

# Menses Phase | Anatomy and Physiology I

## Definition

The menses phase is the part of the menstrual cycle when the endometrium breaks down and is shed as menstrual bleeding. In Anatomy and Physiology I, it is the first phase of the cycle.

## What It Is

The menses phase is the menstrual cycle stage when the uterine lining, called the endometrium, is shed through the vagina as menstrual bleeding. In Anatomy and Physiology I, you usually see it described as the start of a new cycle because day 1 is counted as the first day of bleeding.

This happens because hormone levels drop at the end of the previous cycle, especially estrogen and progesterone. When the corpus luteum stops functioning, the endometrium no longer gets the hormonal support it needs to stay thick and blood-rich, so the superficial layer breaks down and is expelled.

Menses is not the whole menstrual cycle. It is one phase in a larger repeating pattern that includes follicular development, ovulation, and the luteal phase. The bleeding itself usually lasts about 3 to 7 days, though the exact length can vary from person to person and from cycle to cycle.

A useful way to picture it is this: the uterus built up a lining in case pregnancy happened, but if no implantation occurs, that lining is released and the cycle resets. That reset is what makes menses so easy to place on a timeline of the ovarian and uterine cycles.

One common misunderstanding is that menstrual bleeding means the entire lining is gone. It is really the functional layer of the endometrium that is shed, while the deeper basal layer remains so the lining can build back up in the next cycle. That rebuild starts again under the influence of rising estrogen in the follicular phase.

In lab images, class diagrams, and practice questions, menses often shows up as a timing marker. If you can connect bleeding with the hormone drop, the endometrium, and the start of a new cycle, you have the full mechanism instead of just the vocabulary word.

## Why It Matters

Menses phase matters because it is the cleanest sign that the uterus and ovaries are working as a coordinated hormone-controlled system. If you can trace why the lining sheds, you can also trace why the cycle starts over, why estrogen and progesterone change across the month, and how the ovarian and uterine cycles stay in sync.

This term also shows up in a lot of A&P reasoning tasks. A question might describe a person on day 2 of bleeding and ask you to identify the phase, the state of the endometrium, or what the ovaries are likely doing. If you know that menses happens right after hormone withdrawal, you can narrow down the rest of the cycle.

It also connects directly to reproductive health topics. Irregular timing, unusually heavy bleeding, or very short or long cycles can point to disruptions in the hormonal control of the endometrium. Even when the class is not focused on disorders, the normal pattern gives you the baseline for spotting what is off.

Because Anatomy and Physiology I is about structure meeting function, menses is a good example of both. The structure is the uterine lining, and the function is cyclical preparation for pregnancy followed by shedding when pregnancy does not occur.

## Connections

### [Menstrual Cycle](/anatomy-physiology/key-terms/menstrual-cycle)

Menses is one phase of the menstrual cycle, not the entire process. When you place it in the full cycle, you can see what comes before it, like the luteal phase, and what follows it, like follicle growth and endometrial rebuilding. That bigger timeline is often what your instructor wants you to trace.

### Endometrium

The endometrium is the tissue that changes most visibly during menses. During this phase, the functional layer breaks down and is shed, which is why bleeding happens. If you are interpreting a diagram or histology image, the condition of the endometrium tells you where the person is in the cycle.

### Corpus Luteum

The corpus luteum forms after ovulation and secretes hormones that keep the endometrium thick. When it degenerates, hormone levels fall and menses begins. That means the corpus luteum sits right upstream of the phase, even though it is an ovarian structure rather than a uterine one.

### Ovulation

Ovulation does not happen during menses, but it is part of the same repeating cycle. Menses comes first, then follicle development leads to ovulation later in the cycle. If you confuse the two, you can misread timing questions, because bleeding marks the start of the cycle while ovulation happens near the middle.

## On the AP Exam

A quiz question may describe menstrual bleeding and ask you to name the phase, or it may give hormone changes and ask you to predict what the endometrium is doing. You might also see a cycle diagram and need to label the first phase, identify where day 1 starts, or connect falling progesterone to shedding of the lining. In image-based questions, look for a thinner or breaking-down endometrium and match it to menses. In short-answer prompts, use the chain: corpus luteum declines, estrogen and progesterone drop, the endometrium sheds, and a new cycle begins.

## Key Takeaways

- The menses phase is when the endometrium is shed as menstrual bleeding, and it marks day 1 of the menstrual cycle.
- It begins after estrogen and progesterone drop, usually when the corpus luteum is no longer producing enough hormones to maintain the uterine lining.
- Menses is only one part of the cycle, so you should place it before follicular development, ovulation, and the luteal phase on a timeline.
- The bleeding usually lasts about 3 to 7 days, but the exact length can vary from person to person.
- A thin, shedding endometrium points to menses, while a thickened lining points to later phases of the cycle.

## FAQs

### What is menses phase in Anatomy and Physiology I?

The menses phase is the stage of the menstrual cycle when the uterine lining, or endometrium, is shed as menstrual bleeding. In A&P, it is treated as the first day of the cycle. It happens after hormone levels fall and the lining is no longer maintained.

### Is menses phase the same as menstruation?

Yes, they refer to the same basic event: the shedding of the endometrium with menstrual bleeding. In class, you may see either term used. The main thing is to connect it to the start of the cycle and the hormonal drop that triggers it.

### What causes the menses phase to start?

Menses starts when estrogen and progesterone levels fall, usually because the corpus luteum degenerates if pregnancy has not occurred. Without those hormones, the endometrium loses support and breaks down. That shedding is what produces menstrual bleeding.

### How is menses phase different from ovulation?

Menses is the beginning of the cycle and involves shedding the uterine lining. Ovulation happens later, when an oocyte is released from the ovary. If you mix them up, the timing of the whole menstrual cycle gets thrown off.

## Related Study Guides

- [27.2 Anatomy and Physiology of the Ovarian Reproductive System ](/anatomy-physiology/unit-27/2-anatomy-physiology-ovarian-reproductive-system/study-guide/zbASUQTyPUQQ7k6t)

## About This Document

Canonical Fiveable pages are available as Markdown at the same path plus `.md`.

- [llms.txt](https://fiveable.me/llms.txt): index of Fiveable's sections and URL patterns
- [llms-full.txt](https://fiveable.me/llms-full.txt): complete subject and unit listing
- [MCP server](https://fiveable.me/mcp): call Fiveable as tools instead of fetching pages (`https://fiveable.me/api/mcp`)
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