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1. The Feminine Cycle
For a quick revision, see the diagrams of the female reproductive system.
The duration of the feminine cycle is on average 28 days. The first day of bleeding establishes the first day of the cycle.
This cycle is divided into several phases: The Follicular-Stage, Ovulation and then Luteal-Stage.
The follicular stage lasts for about the first 14 or 15 days of the cycle from the first day of the period. During this phase, the release from the ovaries then the maturation of a number of follicles takes place, however only the most mature one of these follicles will produce an egg capable of being fertilised. A follicle is driven to maturity by the secreted hormone FSH (Follicle Stimulating Hormone).
As the follicles mature, they release the hormone estrogen. This makes the uterus wall thicken and the cervical mucus to alter it's consistency.
The Ovulation: As the FSH levels and estrogen levels rise, the secretion of large amounts of another hormone, LH (Leutenising Hormone) is triggered. This rise in LH provokes the most mature follicle to burst open and release an egg from the ovary into the fallopian tube.
As soon as the egg is freed, it is helped along the tube by tiny horn-like fronds that line the fallopian tube. The egg is now in a fertilisable state in a window of about 24 hours.
The graph below shows how the levels of FSH and LH hormones change during a standard 28 day menstruation cycle.
For more detail on the timing of ovulation, see the article on Ovulation Testing
Luteal Stage: This is the stage that leads to the next period. It lasts for about 12 to 14 days. The follicle that released the egg will very quickly become what is known as the "Yellow Body" (in Latin it's name is "Corpus Luteum"). This produces an high level of progesterone (and a small amount of estrogen) which is required to generate the mucus on the uterus surface to assist a fertilised egg.
At the same time, the uterine wall, which is normally very thin, becomes thicker and thicker under the influence of the progesterone secreted by the "yellow body". It is about the 20th day of the cycle that the uterus is prepared to accept an impregnated egg.
If the released egg is impregnated, it will become implanted in the surface of the uterus about 8 days after fertilisation. This implantation will have the effect of releasing a new hormone, HCG. It is this hormone that is tested for with home pregnancy tests.
The HCG allows the "yellow body" to be maintained and continue to produce progesterone and estrogen, assuring the continuation of the pregnancy. This situation remains the same for the first 3 months of pregnancy, the placenta will eventually replace the function of the "yellow body".
If the egg was not fertilised, the "yellow body" stops its activity and degenerates into a "white body" (corpus albican in Latin). The absence of progesterone causes the blood flow to be stopped before reaching the surface of the uterine lining which causes a blood pocket between the surface and the uterus wall that will eventually break the surface lining from the wall and cause the period to begin.
A little point to make...
Young girls are born with all (or usually many more) of the follicles that they will be able to use during a lifetime!
1. The Feminine Cycle
2. The Man's Contribution
3. The Fertilisation
4. How A Girl Or Boy Is Created
5. Marking the Ovulation Time
6. Calculating The Due Date Of Birth
Annex. Female Anatomy
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