Your period is the part of the menstrual cycle when the lining of the uterus breaks down and leaves the body through the vagina. The bleeding happens because hormonal changes at the end of a menstrual cycle cause the uterus to shed tissue it had prepared in case pregnancy occurred.
A period is more than bleeding, however. During those days, the uterus contracts, hormone levels shift, and the body passes blood, mucus, and tissue from the uterine lining. These changes can cause cramps, fatigue, breast tenderness, headaches, bloating, mood changes, and other symptoms.
What happens when your period starts?
A menstrual period begins when levels of the hormones estrogen and progesterone fall after ovulation has not resulted in a pregnancy.
During the previous part of the cycle, these hormones helped thicken the lining of the uterus, called the endometrium. The lining becomes rich in blood vessels and other tissue so it can support an embryo if a fertilized egg implants.
If pregnancy does not occur, the hormonal support for that lining decreases. Small blood vessels in the uterine lining constrict, the tissue begins to break down, and the lining is gradually shed.
The resulting mixture of blood, uterine tissue, mucus, and other fluid passes through the cervix and out through the vagina. This is menstrual bleeding.
Why does the uterus build a lining in the first place?
The uterus prepares for a possible pregnancy during each menstrual cycle.
After a period ends, estrogen produced mainly by developing ovarian follicles helps the uterine lining grow again. The lining becomes thicker and develops the blood supply and cellular environment needed to support implantation.
After ovulation, the ovary produces progesterone, which changes and stabilizes the uterine lining. Progesterone helps prepare it for the possibility that a fertilized egg will implant.
If implantation does not occur, progesterone and estrogen levels eventually fall. Without that hormonal support, the lining can no longer be maintained in the same way, and menstruation begins.
This cycle of building, maintaining, and shedding the uterine lining is why periods occur repeatedly during the reproductive years.
What causes period cramps?
Period cramps are mainly caused by contractions of the uterus.
When menstruation begins, the uterine lining releases substances called prostaglandins. These substances help the muscles of the uterus contract. The contractions help the uterus shed and expel its lining.
Higher levels of prostaglandins are associated with stronger uterine contractions and can contribute to more painful cramps.
The contractions can temporarily reduce blood flow to the uterine muscle, contributing to pain. Some prostaglandins also affect nearby tissues, which helps explain why menstruation can sometimes cause symptoms such as nausea, diarrhea, headaches, or a general feeling of being unwell.
Menstrual cramps are often strongest around the beginning of the period and tend to become less intense as bleeding decreases.
Why can you feel tired or have other symptoms?
Hormonal changes around menstruation can affect many parts of the body.
As estrogen and progesterone levels change, some people experience fatigue, changes in mood, headaches, breast tenderness, bloating, changes in appetite, or difficulty concentrating. The severity and combination of symptoms vary considerably from person to person and can also vary from cycle to cycle.
Some symptoms begin before bleeding starts rather than during the period itself. These are commonly associated with premenstrual syndrome (PMS) and result from the body’s response to hormonal changes during the latter part of the menstrual cycle.
Menstrual bleeding itself can also contribute to fatigue, particularly when bleeding is unusually heavy or prolonged and leads to significant blood loss.
What exactly is menstrual blood?
Menstrual fluid is not simply blood.
It contains blood along with pieces of the uterine lining, cervical mucus, vaginal secretions, and other cellular material. Because it contains different substances, menstrual fluid can have a different appearance and consistency from blood from an ordinary cut.
Its color can range from bright red to dark red or brown. Darker blood often represents blood that has taken longer to leave the uterus and has undergone changes as it was exposed to air.
Small clots can also occur. They form when blood and tissue collect and partially coagulate before leaving the uterus.
Occasional small clots are common, particularly during heavier bleeding. Very large or frequent clots, especially when accompanied by unusually heavy bleeding or significant pain, can be a reason to discuss symptoms with a health care professional.
What happens inside the body during the days of bleeding?
The uterine lining does not come away all at once. It breaks down and is expelled over the course of the menstrual period.
As bleeding continues, the uterus keeps contracting and shedding remaining tissue. At the same time, hormone levels remain relatively low compared with their levels earlier in the cycle.
Meanwhile, new ovarian follicles begin developing under the influence of reproductive hormones. This marks the beginning of the next menstrual cycle even while the current period is still occurring.
As the period ends, estrogen generally begins to rise as an ovarian follicle develops. The uterine lining then starts rebuilding for the next cycle.
What happens after your period ends?
The end of menstrual bleeding does not mean the menstrual cycle has stopped. It marks a transition into the next phase.
During the follicular phase, follicles in the ovaries develop. One follicle typically becomes dominant and continues maturing toward ovulation. Estrogen levels rise as the follicle develops, and the uterine lining begins thickening again.
Eventually, a rise in luteinizing hormone (LH) triggers ovulation, when an egg is released from an ovary.
After ovulation, the cycle enters the luteal phase. The structure left behind by the ovulated follicle produces progesterone, which prepares and maintains the uterine lining for a possible pregnancy.
If pregnancy does not occur, hormone levels fall again, and another period begins.
How long does a period last?
The length of menstrual bleeding varies from person to person and can change over time.
A period commonly lasts several days, but the exact duration is not identical for everyone. The amount of bleeding can also vary, with some people having relatively light bleeding and others experiencing much heavier flow.
The menstrual cycle itself is longer than the period. The menstrual cycle is counted from the first day of one period to the first day of the next. Menstruation is only one phase within that larger cycle.
Cycle length can naturally vary, especially during the first years after menstruation begins and during the years when menstrual cycles are approaching menopause.
Why does bleeding sometimes change from one cycle to another?
Menstrual bleeding is influenced by the hormones that regulate the menstrual cycle, and those hormones can vary from cycle to cycle.
Stress, changes in body weight, illness, intense physical activity, hormonal contraception, pregnancy, breastfeeding, and the transition toward menopause can all affect menstrual patterns.
Some medical conditions can also change the amount, duration, or timing of menstrual bleeding. A noticeable change that persists or repeatedly occurs may therefore deserve medical evaluation rather than being assumed to be a normal variation.
When is period bleeding considered unusually heavy?
Heavy menstrual bleeding is more than simply having a period that feels inconvenient or somewhat heavier than usual. It refers to bleeding that is excessive enough to interfere with daily life or may lead to problems such as anemia.
Signs that bleeding may be unusually heavy include repeatedly soaking through menstrual products very quickly, needing to change protection frequently during the night, passing unusually large clots, bleeding for an unusually long time, or experiencing fatigue or weakness that may be related to blood loss.
Heavy periods can have many causes, including hormonal changes, uterine fibroids, adenomyosis, certain bleeding disorders, medications, and other medical conditions.
When should period pain be checked by a doctor?
Mild to moderate cramping can be a normal part of menstruation, but severe pain is not something that should automatically be accepted as normal.
Medical evaluation is appropriate when menstrual pain is severe, suddenly becomes much worse, consistently interferes with normal activities, begins later in life after previously painless periods, or occurs with other concerning symptoms.
Conditions such as endometriosis, adenomyosis, uterine fibroids, or pelvic infections can cause significant menstrual pain. Identifying the underlying cause can make appropriate treatment possible.
Can you have period symptoms without much bleeding?
Yes. Menstrual symptoms and menstrual bleeding do not always occur to the same degree.
Someone may experience cramps, fatigue, headaches, mood changes, or other symptoms with relatively light bleeding. Hormonal fluctuations and the body’s response to substances such as prostaglandins contribute to symptoms, so the intensity of symptoms does not necessarily match the amount of blood lost.
Similarly, a person can have relatively heavy bleeding with surprisingly little pain.
What does a period tell you about your menstrual cycle?
A period provides one visible sign of the menstrual cycle, but it does not reveal everything happening hormonally.
The first day of menstrual bleeding is considered day one of the menstrual cycle. From there, the ovaries and reproductive hormones move through a coordinated sequence involving follicle development, estrogen production, ovulation, progesterone production, and eventual hormonal decline if pregnancy does not occur.
Understanding this sequence explains why menstruation is not an isolated event. It is the final stage of one reproductive cycle and the starting point of the next.