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Menstrual Cycle

It is the regular natural change that occurs in the female reproductive system (specifically the uterus and ovaries) that makes pregnancy possible.
The cycle is required for the production of ovocytes, and for the preparation of the uterus for pregnancy

The Duration
The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women
- 21 to 35 days in adults (an average of 28 days)
The first period usually begins between 12 & 15 years - Menarche - may occasionally start as early as 8 years, and this onset may still be normal
Menstruation stops occurring after Menopause which usually occurs between 45 and 55 years of age.
Bleeding usually lasts for around 2 to 7 days.

The phases of Menstrual Cycle
The menstrual cycle is governed by hormonal changes.
Each cycle is divided into three phases based on events in the ovary (ovarian cycle) or in the uterus (uterine cycle).

The ovarian cycle
Consists of
  1. Follicular phase
  2. Ovulation phase
  3. Luteal phase

Follicular phase -
The follicular phase is the first part of the ovarian cycle.
The ovarian follicles mature and get ready to release an egg.
The latter part of this phase overlaps with the proliferative phase of the uterine cycle.

During the first days of the cycle a few ovarian follicles are stimulated by the influence of a rise in follicle stimulating hormone (FSH)
These follicles are present at birth and have been developing for about a year - Folliculogenesis
They compete with one another for dominance
Under the influence of several  hormones only one follicle matures and the others stop growing
The one follicle which matures is called Graafian follicle and it contains the ovum

Ovulation phase -
Ovulation is the second phase of the ovarian cycle in which a mature egg is released from the ovarian follicles into the oviduct.
During the follicular phase, estradiol suppresses production of luteinizing hormone (LH) from the anterior pituitary gland.
When the egg has nearly matured, levels of estradiol reach a threshold above which this effect is reversed and estrogen stimulates the production of a large amount of LH.
This process, known as the LH surge, starts around day 12 of the average cycle and may last 48 hours.
The release of LH matures the egg and weakens the wall of the follicle in the ovary, causing the fully developed follicle to release its secondary oocyte.
The secondary oocyte promptly matures into an ootid and then becomes a mature ovum.

Which of the two ovaries—left or right—ovulates appears essentially random; no known left and right co-ordination exists
Occasionally, both ovaries will release an egg;
If both eggs are fertilized, the result is fraternal twins.

After being released from the ovary, the egg is swept into the fallopian tube by the fimbria,
After about a day, an unfertilized egg will disintegrate or dissolve in the fallopian tube.

Fertilization by a spermatozoon, when it occurs, usually takes place in the ampulla
A fertilized egg immediately begins the process of embryogenesis.
The developing embryo takes about three days to reach the uterus and another three days to implant into the endometrium.
It has usually reached the blastocyst stage at the time of implantation.

In some women, ovulation features a characteristic pain called mittelschmerz
The sudden change in hormones at the time of ovulation sometimes also causes light mid-cycle blood flow.

Luteal phase
The third and final phase of the ovarian cycle
It corresponds to the secretory phase of the uterine cycle.
During the luteal phase, the pituitary hormones FSH and LH cause the remaining parts of the dominant follicle to transform into the corpus luteum, which produces progesterone.
The increased progesterone in the adrenals starts to induce the production of estrogen.
The hormones produced by the corpus luteum also suppress production of the FSH and LH that the corpus luteum needs to maintain itself. Consequently, the level of FSH and LH fall
Corpus luteum atrophies.
Falling levels of progesterone trigger menstruation and the beginning of the next cycle.
The loss of the corpus luteum is prevented by fertilization of the egg.
The syncytiotrophoblast, which is the outer layer of the resulting embryo (the blastocyst), and later also becomes the outer layer of the placenta.
The syncytiotrophoblast produces human chorionic gonadotropin (hCG), which is very similar to LH and which preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.

The uterine cycle
is divided into
Menstruation phase
Proliferative phase
Secretory phase.

The flow of blood normally serves as a sign that a woman has not become pregnant.
Eumenorrhea/normal/regular menstruation  lasts for a few days (usually 3 to 5 days, but anywhere from 2 to 7 days is considered normal). The average blood loss : 10–80 ml considered normal.
An enzyme called plasmin inhibits clotting in the menstrual fluid.
Painful cramping in the abdomen, back, or upper thighs is common during the first few days of menstruation.
Severe uterine pain during menstruation is known as dysmenorrhea, and it is most common among adolescents and younger women
When menstruation begins, symptoms of premenstrual syndrome (PMS) such as breast tenderness and irritability generally decrease.

Proliferative phase
Estrogen causes the lining of the uterus to grow, or proliferate.
As they mature, the ovarian follicles secrete increasing amounts of estradiol, and estrogen.
The estrogens initiate the formation of a new layer of endometrium in the uterus.
Estrogen also stimulates crypts in the cervix to produce fertile cervical mucus, which may be noticed by women practicing fertility awareness.

Secretory phase
It corresponds to the luteal phase of the ovarian cycle.
During the secretory phase, the corpus luteum produces progesterone.
It makes the endometrium receptive to implantation of the blastocyst and supportive of the early pregnancy, by increasing blood flow and uterine secretions and reducing the contractility of the smooth muscle in the uterus.
It  raises the woman's basal body temperature.

Importance of ovulation time
It is the time when a woman can become pregnant
It is at that time that the couple should have intercourse
The ovulation time is marked by a rise in the basal body temperature
It is marked by a vaginal discharge of mucus which is fertile : a change in the cervical mucus can be detected by examining the cervical mucus
Ovulation takesplace about 14 days before the next menstrual period.
The normal menstrual cycle is calculated to 28 days. Ovulation takes place on the 14th day and the fertile window begins on the 12th day.
In a long 35 days menstrual cycle ovulation takes place on the 21st day and fertile window begins on the 19thday. 
In a short 21 days menstrual cycle ovulation takes place on the 7th day and the fertile window begins on the 5th day.
The ovulation time is fairly constant if calculated from the last day of the cycle.

Mention the normal duration of menstrual cycle. Describe the phases of menstrual cycle. Discuss the importance of ovulation time.

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