The menstrual cycle
During the menstrual cycle, a woman’s body undergoes multiple changes in preparation for the possibility of pregnancy. This hormonal process typically happens every month and includes ovulation and, if pregnancy does not occur, a menstrual period.
The main organs involved in the menstrual cycle are the uterus and the ovaries. These organs progress through different stages in each menstrual cycle.
The uterus is the muscular, hollow organ that expands to support pregnancies. The lower portion includes the cervix, which opens into the vagina. The upper portion connects to the fallopian tubes. The fallopian tubes provide a path for eggs released from the ovaries to travel to the uterus.
The inner lining of the uterus is called the endometrium. During pregnancy, the endometrium becomes part of the placenta. If pregnancy does not occur, the endometrium sheds in a menstrual period.
Several follicles develop in each menstrual cycle, each one holding an immature egg. In ovulation, a follicle ruptures and an egg is released.
The menstrual cycle consists of three, interconnected cycles: the uterine cycle, ovarian cycle, and hormone cycle. The rise and fall of hormones cause the observed changes in the uterus and the ovaries.
In the proliferative phase, the endometrium grows in response to rising estrogen levels. In the secretory phase, the endometrium undergoes additional changes to prepare for pregnancy in response to progesterone. In the menstrual phase, the endometrium is shed in response to a decrease in progesterone.
In the follicular phase, the ovarian follicles begin to grow and prepare for ovulation in response to follicle-stimulating hormone (FSH). Ovulation occurs when a follicle ruptures and releases an egg in response to luteinizing hormone (LH). In the luteal phase, the remnant of the follicle forms the corpus luteum, which produces progesterone and prepares the endometrium for possible pregnancy.
FSH and LH are released by the anterior pituitary. Estrogen and progesterone are released by the ovaries. All of these hormones work together to trigger each phase of the uterine and ovarian cycles.
When is it not normal?
What is normal can vary between women. For example, the average length of the menstrual cycle is 28 days, but can range from 21 to 35 days. For most women, menstruation lasts 5 to 7 days. More research on the menstrual cycle is needed, but certain symptoms may indicate underlying disease.
A typical amount of blood loss during menstruation is 35 to 50 mL (2 to 3 tablespoons). Heavy bleeding is more than 80 mL (approximately 6 tablespoons) of blood loss in a cycle.
Many women experience some discomfort during their menstrual cycles. Moderate-to-severe pain that disrupts daily activities is not normal.