Events of the menstrual cycle occur in the uterus in preparation for a possible pregnancy. The menstrual cycle runs simultaneously with the ovarian cycle. Both occur in response to rising and falling hormone secretions and result in menstruation or the cyclical release of the uterine lining.

If pregnancy does not occur, menstruation, menstrual flow, or menses happen in response to decreasing estrogen and progesterone concentrations—menstrual flow forms from degraded uterine endometrial tissue and fluid. Once enough accumulates, constricting uterine blood vessels and smooth muscle contractions allow menstruation. The onset of menstruation occurs during puberty around the ages of 10 to 16 and ceases on average around 51 years old.

The menstrual cycle begins with day one of menstrual flow. The average duration of menstrual flow ranges from three to seven days but can vary from two to eight days and be considered normal. The average menstrual cycle length is 28 days, with most cycles ranging between 25 to 30 days. The average menstrual blood loss is 30 ml, with amounts greater than 80 mL considered abnormal.

Menorrhagia is when blood loss is greater than 80 mL in one cycle. Amenorrhea is defined by the absence of menstruation. Oligomenorrhea or infrequent menstruation occurs when menstrual cycles are longer than 35 days and are often associated with PCOS. Dysmenorrhea or painful menstrual cramps are related to hormonal imbalances, increased production of prostaglandins, and is associated with endometriosis. Premenstrual disorders including premenstrual syndrome and premenstrual dysphoric disorder, are accompanied by various physical and psychological symptoms.

Understanding the menstrual cycle is imperative because menstrual cycle irregularities may indicate underlying medical conditions and reveal ovulatory dysfunction. Menstrual cycle irregularities, including the variations in cycle length, menstrual flow, and premenstrual symptoms, provide insight into menstrual disorders, ovulatory dysfunction, and health risks. Please seek medical treatment from a qualified health professional if you are experiencing abnormal menstruation.

Uterine changes occur throughout the menstrual cycle in response to estrogen and progesterone plasma concentrations. There are three phases of the uterine cycle: menstrual, proliferative, and secretory phases.

The menstrual phase begins on day one of menstrual flow and extends throughout menstruation. During this time, the degraded endometrium is released through menstrual flow.

After menstruation, the proliferative phase begins as the endometrium thickens under the influence of estrogen. This period of growth lasts about ten days or until ovulation.

After ovulation, the endometrium develops coiled glands and more blood vessels. During this time, the endometrium secretes glycogen, glycoproteins, and mucopolysaccharides in response to progesterone and estrogen. These changes in the endometrium are essential for the implantation and nourishment of a developing embryo.

The ovarian cycle describes the events in the ovaries and includes the follicular phase, ovulation, and the luteal phase. The menstrual cycle runs simultaneously with the ovarian cycle, as both occur in response to hormones produced by the hypothalamic-pituitary-ovarian (HPO) axis.

The hypothalamic-pituitary-ovarian (HPO) axis is a complex communication system between the hypothalamus, anterior pituitary, and ovaries. The HPO axis regulates the ovarian and menstrual cycles through the release of gonadotropin-releasing hormone (GnRH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and sex hormones estrogen and progesterone.

The follicular phase begins with the first day of menstruation and continues until ovulation, or in other words, from day one to 14 of a 28-day menstrual cycle. Early in the follicular phase, multiple follicles develop and compete for FSH, estrogen production, and ultimately, for which will be the dominant follicle.

Ovulation occurs when the mature follicle ruptures and releases the oocyte or egg from the ovary. The time of ovulation varies. In a typical 28-day cycle, ovulation occurs around day 14. Leading up to ovulation, a large amount of estrogen is secreted from the dominant follicle, causing a significant peak in plasma concentrations of estrogen. In response, the hypothalamus and anterior pituitary increase LH secretions, known as the LH surge. The LH surge induces ovulation. Meanwhile, the ciliated fimbriae of the fallopian tubes gently sweep the released oocyte into the fallopian tube towards the uterus.

Some experience signs of ovulation, including changes in cervical mucus, rising basal body temperature post-ovulation, and an ovarian twinge of pain called mittelschmerz, German for “middle pain.”

If fertilization does not occur, the luteal phase begins immediately after ovulation and ends at the onset of menstruation or from day 15 to 28 in a 28-day cycle. After ovulation, the ruptured follicle is transformed into the corpus luteum, or “yellow-body” in Latin, resulting from the cellular accumulation of lipids. The function of the corpus luteum is to secrete significant amounts of progesterone and some estrogen to prepare the endometrium of the uterus for pregnancy. If the egg is not fertilized, the lifespan of the corpus luteum is short, as it will degrade within two weeks.

If the egg is fertilized within 24 to 48 hours of ovulation and pregnancy occurs, the early placenta will produce human chorionic gonadotropin (hCG), preserving the corpus luteum until the placenta can make sufficient progesterone. HCG secretion peaks around 60 to 80 days after the last menstruation. It is followed by a rapid decrease of hCG, in which the placenta produces large amounts of progesterone and estrogen, allowing the corpus luteum to regress after three months.