Back To Basics: What’s Really Going on During Your Period?

Authors: Joshua Dexheimer, PhD and Nicole Strock, PhD

The menstrual cycle is an important indicator of overall health for women (it’s even considered the fifth vital sign). [1] And it’s thankfully getting its moment in the spotlight.

More and more women are aware that their menstrual cycle not only reveals a lot about their reproductive health, but also their day-to-day energy levels, food preferences, mood, and cognition (think brain fog). New technology is helping women better navigate the twists and turns of the menstrual cycle, with some devices helping track what cycle phase and even manage undesirable side effects like period pain.

But before jumping straight to the technology, it’s important to understand the four phases of the menstrual cycle, what’s really going on during your period, and why women experience period pain.

What Is the Menstrual Cycle?

The menstrual cycle refers to the sequence of hormonal events that occur within the body as it prepares for a possible pregnancy. [2] It starts on the first day of menses — your period — until the first day of your next period.

The length of this cycle varies for each person. It typically lasts an average of 28 days, but it’s normal to span anywhere from 21 to 35 days. [3] It’s composed of four distinct phases: the menstrual phase, follicular phase, ovulation phase, and luteal phase.

Each phase is driven by fluctuations in different sex hormones — namely estrogen, progesterone, luteinizing hormone (LH), and follicle-stimulating hormone (FSH). Shifts in these hormones can result in period-associated symptoms like bloating, headaches, and cramping. Here’s what goes on during each phase of a period.

Menstrual Phase

Your period is the first stage of the menstrual cycle when pregnancy doesn’t occur. Estrogen, progesterone, and luteinizing hormone levels drop, telling the uterus to shed the lining (of blood, tissue, and mucous) it built up to support a potential pregnancy.

Some women may experience period cramps and pain, mood swings, and tender breasts, whereas others seemingly glide through it. [4]

Follicular Phase

The follicular phase actually begins on the first day of your period (so there is some overlap in the timeline here). During this stage, an egg matures in an ovary, and an increase in estrogen tells the uterus to begin to prepare again for a potential pregnancy. On average, the follicular phase lasts about 16 days, ending with ovulation. [5]

Ovulation

When luteinizing hormone and a follicle reach their peak concentration in the body, they trigger ovulation — or the release of an egg from an ovary. The egg has about 12 to 24 hours to be fertilized, otherwise it dissolves. This is the point in the menstrual cycle where women are most fertile (or likely to become pregnant). Most women physically feel the best during this phase of the cycle — their mood is more positive, and symptoms are less present. [4]

Luteal Phase

Next is the luteal phase. During this time, the ruptured follicle (from which the egg was released) transforms into a structure called the corpus luteum. The corpus luteum produces the hormone progesterone that tells the uterus to keep prepping for a potential pregnancy (i.e., thickening the uterine wall). [6]

If fertilization of the egg does not occur, the corpus luteum breaks down, progesterone levels drop (as do estrogen levels), and then the cycle starts once again with menstruation.

The luteal phase is about 14 days and is the most notable time for premenstrual symptoms (PMS) like bloating, fatigue, irritability, feelings of anxiousness, and headaches. These symptoms can also run into the menstrual phase. [7]

Women often report the most severe mood and strongest PMS effects during either the luteal phase or menstruation. [4]