The menstrual phase, commonly known as a period, marks the beginning of the menstrual cycle. This phase starts when an egg from the previous cycle is not fertilized. In response, the body sheds the thickened lining of the uterus, which exits the body as a discharge of blood, uterine tissue, and mucus.
The Underlying Biological Process
The menstrual phase is a hormonal event initiated by a sharp decline in estrogen and progesterone. These hormones were responsible for building up and maintaining the uterine lining, called the endometrium, in preparation for a potential pregnancy. When fertilization does not happen, this hormonal support is withdrawn.
This drop in hormones causes the endometrium to break down and detach from the uterine wall, which constitutes the menstrual flow. The process serves as a physiological reset, preparing the uterus for the next cycle. This shedding is accompanied by uterine cramps, caused by contractions of the uterine muscles to help expel the lining.
As the old lining is shed, a new process is already beginning. The hypothalamus signals the pituitary gland to release follicle-stimulating hormone (FSH). This hormone travels to the ovaries and encourages the development of several follicles, each containing an immature egg. This overlap ensures that as one cycle concludes, preparations for the next are underway.
Common Physical and Emotional Experiences
The experience of the menstrual phase varies greatly, but many report a predictable set of physical symptoms. Beyond uterine cramps, other common physical experiences include bloating, breast tenderness, headaches, and general fatigue. Some people also experience low back pain that can range from a dull ache to more intense discomfort.
Alongside physical changes, emotional shifts are also a frequent part of the menstrual phase. The hormonal fluctuations, particularly the drop in estrogen and progesterone, can influence neurotransmitters in the brain. This can lead to noticeable changes in mood, such as increased irritability or feeling more withdrawn and less social.
The intensity and combination of these symptoms are highly individual. One person may experience significant cramping but stable moods, while another might have minimal physical discomfort but feel emotionally sensitive. These experiences can also change from one cycle to the next, influenced by factors like stress, diet, and overall health.
Duration and Flow Variations
The length of the menstrual phase itself is not the same for everyone. A typical, or average, duration for bleeding is between three and seven days. While some individuals may consistently have shorter periods of around three days, others may regularly bleed for a full week, and both are considered within a normal range. Cycles are generally considered regular when the variation between the longest and shortest cycle is less than eight days.
The volume of menstrual flow also differs significantly. The average amount of blood lost during an entire period is about two to three tablespoons, or approximately 35 mL. What is considered a normal flow for one person may be light or heavy for another. Signs of unusually heavy bleeding could include needing to change a pad or tampon every one to two hours or passing blood clots larger than the size of a quarter.
Factors such as age can influence the duration and flow of menstruation. For instance, when periods first begin during the teenage years, cycles may be longer and flow heavier. It can take a few years for cycles to settle into a more regular pattern. Similarly, stress and other lifestyle factors can temporarily alter the length and character of the menstrual phase from month to month.
Context Within the Full Menstrual Cycle
The menstrual phase is the first part of the larger menstrual cycle, which is a month-long process of preparing the body for a potential pregnancy. Specifically, the menstrual phase initiates the follicular phase. The follicular phase begins on the first day of bleeding and continues until ovulation, meaning it overlaps entirely with the menstrual phase and extends beyond it.
Once the bleeding of the menstrual phase stops, the follicular phase continues with the primary goal of maturing an egg. During this time, rising estrogen levels cause the uterine lining to begin thickening again, preparing a new environment for a potential embryo. This phase is driven by follicle-stimulating hormone (FSH), which encourages the growth of ovarian follicles until one becomes dominant.
Following the follicular phase, the cycle progresses to ovulation, typically around day 14 in a 28-day cycle, when the mature egg is released from the ovary. This is triggered by a surge in luteinizing hormone (LH). The final stage is the luteal phase, where progesterone levels rise to further prepare the uterine lining. If the egg is not fertilized, hormone levels fall, the lining breaks down, and the entire cycle begins again with the menstrual phase.