The menstrual cycle is a repetitive sequence of events that occurs in fertile women roughly every 28 days. The endometrium (i. e. the inner layer of the uterine wall) renews and gets ready for accepting a fertilized egg - the menstrual cycle is therefore essential for pregnancy to happen. It is close to the ovarian cycle but not the same - while the ovarian cycle includes the eggs maturation and histological and hormonal changes in the ovaries, the menstrual cycle refers to changes of the endometrium and to the menstruation itself.
Timing
Menarche, the first period, usually begins between twelve and fifteen years of age (the average age is generally later in the developing countries than in the developed world). Factors such as genetics, diet, and overall health can affect timing. The typical length of the menstrual cycle, i. e. the time between the first day of one and the first day of the next period, is on average 28 days. However, this varies among women, and even in the same woman from one cycle to the next, typically from 21 to 35 days. Bleeding usually lasts around 2 to 7 days. The cessation of menstrual cycles at the end of a woman's reproductive period is called menopause. After that, a woman can no longer get pregnant.
Phases
The production of hormones by the ovaries (the granulosa and theca cells) throughout the ovarian cycle controls the events in the uterus. This results in three phases of the menstrual cycle: the menses phase, the proliferative phase, and the secretory phase.
1. Menses phase
During this phase, the endometrium is shed, which results in the menstruation (i. e. shedding of blood and pieces of endometrial tissue through the vagina). Although it averages approximately 5 days, the menses phase can last from 2 to 7 days, or even longer. The menses phase occurs during the early days of the follicular phase of the ovarian cycle, when progesterone, follicle stimulating hormone (FSH), and luteinizing hormone (LH) levels are low (Pic. 1). As the corpus luteum (a progesterone producing structure that forms in the ovary after ovulation, i. e. when the oocyte is released, from the rest of the follicle: the granulose and theca cells; Pic. 2) degrades, the progesterone concentrations decline. This marks the end of the luteal phase of the ovarian cycle. This decline in progesterone triggers the shedding of the stratum functionalis of the endometrium.
2. Proliferative phase
Once menstrual flow ceases, the endometrium begins to proliferate (i. e. the cells multiply) again, marking the beginning of the proliferative phase of the menstrual cycle. It occurs when the granulosa and theca cells of the follicles begin to produce increased amounts in estrogen. These rising estrogen concentrations stimulate the endometrial lining to rebuild. Ovulation marks the end of the proliferative phase of the menstrual cycle as well as the end of the follicular phase of the ovarian cycle. Besides inducing the ovulation, high estrogen levels also increase the uterine tube contraction (that facilitate the pick-up and transfer of the ovulated oocyte), and decrease the acidity of the vagina, making it more hospitable to sperm.
3. Secretory phase
After the ovulation, the corpus luteum comes ingo being in the ovary. In the uterus, progesterone from the corpus luteum begins the secretory phase. In this phase, the endometrial lining prepares for implantation (Pic. 3). Over the next 10 to 12 days, the endometrial glands secrete a fluid rich in glycogen. If fertilization has occurred, this fluid will nourish the ball of cells now developing from the zygote (Pic. 4). At the same time as this secretion, the spiral arteries develop to provide blood to the thickened stratum functionalis (i. e. the uppermost layer of the endometrium).
If no pregnancy occurs within approximately 10 to 12 days, the corpus luteum will degrade into the corpus albicans (Pic. 5). Levels of both estrogen and progesterone will fall, and the endometrium will grow thinner. The spiral arteries will constrict, therefore the oxygen supply will decrease and the endometrial tissue will die. This will result in menses - or the first day of the next cycle.
Menstrual cycle and fertility
The most fertile period covers the time from some 5 days before until 1 to 2 days after ovulation. In an average cycle, this corresponds to the second and the beginning of the third week. A variety of methods have been developed to help individual women estimate the relatively fertile and relatively infertile days in the cycle - for example urine test kits that detect the LH surge occuring 24 to 36 hours before ovulation ("ovulation predictor kits"), computerized devices that interpret basal body temperatures, or urinary test results.
As measured on women undergoing in vitro fertilisation (IVF), a longer menstrual cycle length is associated with higher pregnancy and delivery rates, even after age adjustment. Delivery rates after IVF have benn estimated to be almost doubled for women with a menstrual cycle length of more than 34 days compared with women with a menstrual cycle length shorter than 26 days. A longer menstrual cycle length is also significantly associated with better ovarian response to gonadotropin stimulation (i. e. administration of follicle stimulating and luteinizing hormone to a woman undergoing assisted reproduction in order to stimulate her ovarian function, thus facilitating the development of more oocytes) and embryo quality.
Menstrual cycle and areas of life
Changing levels of hormones can affect various aspects of a woman's life. This may include mood and behaviour (empathy, emotion recognition, fear response, sexual feelings...), mate choice (preference in different voice pitch, body odor, facial features... this affects women seeking short-term partners more than women seeking long-term partners), or eating behavior.
The menstrual cycle can be affected by various menstrual disorders - see a separate article Menstrual cycle disorders.
