About Cycles
The menstrual cycle is the set of recurring physiological changes in a female's body that are under the control of the reproductive hormone system and necessary for reproduction. In women, menstrual cycles occur typically on a monthly basis between puberty and menopause. Besides humans, only other great apes exhibit menstrual cycles, in contrast to the estrus cycle of most mammalian species.
During the menstrual cycle, the sexually mature female body releases one egg (or occasionally two, which might result in dizygotic, or non-identical, twins) at the time of ovulation. The lining of the uterus, the endometrium, builds up in a synchronised fashion. After ovulation, this lining changes to prepare for potential implantation of the fertilised egg to establish a pregnancy. If fertilisation and pregnancy do not ensue, the uterus sheds the lining and a new menstrual cycle begins. The process of the shedding of the lining is called menstruation. Menstruation manifests itself to the outer world in the form of the menses (also menstruum): essentially part of the endometrium and blood products that pass out of the body through the vagina. Although this is commonly referred to as blood, it differs in composition from venous blood.
Common usage refers to menstruation and menses as a period. This bleeding serves as a sign that a woman has not become pregnant. (However, this cannot be taken as certainty, as sometimes there is some bleeding in early pregnancy.) During the reproductive years, failure to menstruate may provide the first indication to a woman that she may have become pregnant. A woman might say that her "period is late" when an expected menstruation has not started and she might have become pregnant.
Menstruation forms a normal part of a natural cyclic process occurring in healthy women between puberty and the end of the reproductive years. The onset of menstruation, known as menarche, occurs at an average age of 12, but can occur any time between the ages of 8 and 16. However, the condition precocious puberty has caused menstruation to occur in girls as young as 8 months old. The last period, menopause, usually occurs between the ages of 45 and 55. Deviations from this pattern deserve medical attention. Amenorrhea refers to a prolonged absence of menses during the reproductive years of a woman for reasons other than pregnancy. For example, women with very low body fat, such as athletes, may cease to menstruate. The presence of menstruation does not prove that ovulation took place; women who do not ovulate may have menstrual cycles. Those anovulatory cycles tend to take place less regularly and show greater variation in cycle length. In addition, the absence of menstruation also does not prove that ovulation did not take place, because hormone disruptions in non-pregnant women can suppress bleeding on occasion.
The normal menstrual cycle in humans
Women show considerable variation in the lengths of their menstrual cycles, and the length of the menstrual cycle differs in different animals (see below).
While cycle length may vary, 28 days is generally taken as representative of the average ovulatory cycle in women. Convention uses the onset of menstrual bleeding to mark the beginning of the cycle, so the first day of bleeding is called "Cycle Day one".
One can divide the menstrual cycle into four phases:
Menstruation
Menstruation lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal) and involves the loss of about 30-40 millilitres of blood; many women also notice shed endometrium lining that appears as tissue mixed with the blood. An enzyme called plasmin — contained in the endometrium — inhibits the blood from clotting. Because of this blood loss, women have higher dietary requirements for iron than do males to prevent iron deficiency. Many women experience uterine cramps, also referred to as dysmenorrhea, during this time. A vast industry has grown to provide sanitary products to help women to manage their menses.
Follicular phase
Through the influence of a rise in Follicle stimulating hormone (FSH), five to seven tertiary-stage ovarian follicles are recruited for entry into the menstrual cycle. These follicles, that have been growing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance. In a signal cascade kicked off by luteinizing hormone (LH), the follicles secrete estradiol, a steroid that acts to inhibit pituitary secretion of FSH. With diminished FSH supply comes a slowing in growth that eventually leads to follicle death, known as atresia. The largest follicle secretes inhibin that serves as a finishing blow to less competent follicles by further suppressing FSH. This dominant follicle continues growing, forms a bulge near the surface of the ovary, and soon becomes competent to ovulate.
The follicles also secrete estrogens (of which estradiol is a member). Estrogens initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium. If fertilised, the embryo will implant itself within this hospitable flesh.
Ovulation
This ovary is about to release an egg.
When the follicle has matured, it secretes enough estradiol to trigger the acute release of luteinizing hormone (LH). In the average cycle this LH surge starts around cycle day 12 and may last 48 hours. The release of LH matures the egg and weakens the wall of the follicle in the ovary. This process leads to ovulation: the release of the now mature ovum, the largest cell of the body (with a diameter of about 0.5 mm). Which of the two ovaries — left or right — ovulates appears essentially random; no known left/right co-ordination exists. The Fallopian tube needs to capture the egg and provide the site for fertilisation. A characteristic clear and stringy mucus exhibiting spinnbarkeit develops at the cervix, ready to accept sperm from intercourse. In some women, ovulation features a characteristic pain called Mittelschmerz which lasts for several hours. The sudden change in hormones at the time of ovulation also causes light mid-cycle bleeding for some women. Many women perceive the vaginal and cervical mucus changes at ovulation, particularly if they are monitoring themselves for signs of fertility. An unfertilised egg will eventually disintegrate or dissolve in the uterus. Scientific investigations have indicated that the olfactory acuity or the sense of smell is greatest during ovulation in women.
Luteal phase
The corpus luteum is the solid body formed in the ovaries after the egg has been released from the fallopian tube which continues to grow and divide for a while. After ovulation, the residual follicle transforms into the corpus luteum under the support of the pituitary hormones. This corpus luteum will produce progesterone in addition to estrogens for approximately the next 2 weeks. Progesterone plays a vital role in converting the proliferative endometrium into a secretory lining receptive for implantation and supportive of the early pregnancy. It raises the body temperature by half- to one degree Fahrenheit (one-quarter to one-half degree Celsius), thus women who record their temperature on a daily basis will notice that they have entered the luteal phase. If fertilisation of an egg has occurred, it will travel as an early embryo through the tube to the uterine cavity and implant itself 6 to 12 days after ovulation. Shortly after implantation, the growing embryo will signal its existence to the maternal system. One very early signal consists of human chorionic gonadotropin (hCG), a hormone that pregnancy tests can measure. This signal has an important role in maintaining the corpus luteum and enabling it to continue to produce progesterone. In the absence of a pregnancy and without hCG, the corpus luteum demises and inhibin and progesterone levels fall. This will set the stage for the next cycle. Progesterone withdrawal leads to menstrual shedding (progesterone withdrawal bleeding), and falling inhibin levels allow FSH levels to rise to raise a new crop of follicles.
During the menstrual cycle, the sexually mature female body releases one egg (or occasionally two, which might result in dizygotic, or non-identical, twins) at the time of ovulation. The lining of the uterus, the endometrium, builds up in a synchronised fashion. After ovulation, this lining changes to prepare for potential implantation of the fertilised egg to establish a pregnancy. If fertilisation and pregnancy do not ensue, the uterus sheds the lining and a new menstrual cycle begins. The process of the shedding of the lining is called menstruation. Menstruation manifests itself to the outer world in the form of the menses (also menstruum): essentially part of the endometrium and blood products that pass out of the body through the vagina. Although this is commonly referred to as blood, it differs in composition from venous blood.
Common usage refers to menstruation and menses as a period. This bleeding serves as a sign that a woman has not become pregnant. (However, this cannot be taken as certainty, as sometimes there is some bleeding in early pregnancy.) During the reproductive years, failure to menstruate may provide the first indication to a woman that she may have become pregnant. A woman might say that her "period is late" when an expected menstruation has not started and she might have become pregnant.
Menstruation forms a normal part of a natural cyclic process occurring in healthy women between puberty and the end of the reproductive years. The onset of menstruation, known as menarche, occurs at an average age of 12, but can occur any time between the ages of 8 and 16. However, the condition precocious puberty has caused menstruation to occur in girls as young as 8 months old. The last period, menopause, usually occurs between the ages of 45 and 55. Deviations from this pattern deserve medical attention. Amenorrhea refers to a prolonged absence of menses during the reproductive years of a woman for reasons other than pregnancy. For example, women with very low body fat, such as athletes, may cease to menstruate. The presence of menstruation does not prove that ovulation took place; women who do not ovulate may have menstrual cycles. Those anovulatory cycles tend to take place less regularly and show greater variation in cycle length. In addition, the absence of menstruation also does not prove that ovulation did not take place, because hormone disruptions in non-pregnant women can suppress bleeding on occasion.
The normal menstrual cycle in humans
Women show considerable variation in the lengths of their menstrual cycles, and the length of the menstrual cycle differs in different animals (see below).
While cycle length may vary, 28 days is generally taken as representative of the average ovulatory cycle in women. Convention uses the onset of menstrual bleeding to mark the beginning of the cycle, so the first day of bleeding is called "Cycle Day one".
One can divide the menstrual cycle into four phases:
Menstruation
Menstruation lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal) and involves the loss of about 30-40 millilitres of blood; many women also notice shed endometrium lining that appears as tissue mixed with the blood. An enzyme called plasmin — contained in the endometrium — inhibits the blood from clotting. Because of this blood loss, women have higher dietary requirements for iron than do males to prevent iron deficiency. Many women experience uterine cramps, also referred to as dysmenorrhea, during this time. A vast industry has grown to provide sanitary products to help women to manage their menses.
Follicular phase
Through the influence of a rise in Follicle stimulating hormone (FSH), five to seven tertiary-stage ovarian follicles are recruited for entry into the menstrual cycle. These follicles, that have been growing for the better part of a year in a process known as folliculogenesis, compete with each other for dominance. In a signal cascade kicked off by luteinizing hormone (LH), the follicles secrete estradiol, a steroid that acts to inhibit pituitary secretion of FSH. With diminished FSH supply comes a slowing in growth that eventually leads to follicle death, known as atresia. The largest follicle secretes inhibin that serves as a finishing blow to less competent follicles by further suppressing FSH. This dominant follicle continues growing, forms a bulge near the surface of the ovary, and soon becomes competent to ovulate.
The follicles also secrete estrogens (of which estradiol is a member). Estrogens initiate the formation of a new layer of endometrium in the uterus, histologically identified as the proliferative endometrium. If fertilised, the embryo will implant itself within this hospitable flesh.
Ovulation
This ovary is about to release an egg.
When the follicle has matured, it secretes enough estradiol to trigger the acute release of luteinizing hormone (LH). In the average cycle this LH surge starts around cycle day 12 and may last 48 hours. The release of LH matures the egg and weakens the wall of the follicle in the ovary. This process leads to ovulation: the release of the now mature ovum, the largest cell of the body (with a diameter of about 0.5 mm). Which of the two ovaries — left or right — ovulates appears essentially random; no known left/right co-ordination exists. The Fallopian tube needs to capture the egg and provide the site for fertilisation. A characteristic clear and stringy mucus exhibiting spinnbarkeit develops at the cervix, ready to accept sperm from intercourse. In some women, ovulation features a characteristic pain called Mittelschmerz which lasts for several hours. The sudden change in hormones at the time of ovulation also causes light mid-cycle bleeding for some women. Many women perceive the vaginal and cervical mucus changes at ovulation, particularly if they are monitoring themselves for signs of fertility. An unfertilised egg will eventually disintegrate or dissolve in the uterus. Scientific investigations have indicated that the olfactory acuity or the sense of smell is greatest during ovulation in women.
Luteal phase
The corpus luteum is the solid body formed in the ovaries after the egg has been released from the fallopian tube which continues to grow and divide for a while. After ovulation, the residual follicle transforms into the corpus luteum under the support of the pituitary hormones. This corpus luteum will produce progesterone in addition to estrogens for approximately the next 2 weeks. Progesterone plays a vital role in converting the proliferative endometrium into a secretory lining receptive for implantation and supportive of the early pregnancy. It raises the body temperature by half- to one degree Fahrenheit (one-quarter to one-half degree Celsius), thus women who record their temperature on a daily basis will notice that they have entered the luteal phase. If fertilisation of an egg has occurred, it will travel as an early embryo through the tube to the uterine cavity and implant itself 6 to 12 days after ovulation. Shortly after implantation, the growing embryo will signal its existence to the maternal system. One very early signal consists of human chorionic gonadotropin (hCG), a hormone that pregnancy tests can measure. This signal has an important role in maintaining the corpus luteum and enabling it to continue to produce progesterone. In the absence of a pregnancy and without hCG, the corpus luteum demises and inhibin and progesterone levels fall. This will set the stage for the next cycle. Progesterone withdrawal leads to menstrual shedding (progesterone withdrawal bleeding), and falling inhibin levels allow FSH levels to rise to raise a new crop of follicles.
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