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Phases of the Menstrual Cycle
9. May, 2011
As puberty begins for young girls (usually between 8 and 13 years old), the hormones which stimulate the physical characteristics of their adult body also trigger menstruation. Menarche is the term used to describe the first period or first menstrual bleeding in a woman’s life and is used as the indicator that she now has the potential of fertility; the first menstrual bleeding shows the reproductive organs are mature and functioning together appropriately.
What does that really mean?
An ovarian cycle is easily broken into three phases, and should not be confused with the uterine cycle though both are closely timed and interlinked. The three phases are Menstrual phase, Proliferative phase and Secretory phase.
The first phase is called the menstrual phase, it is heralded by the first day of bleeding (menstruation or period) and is also the first day of the ovarian cycle. All of the hormones, primarily estrogen, progesterone, Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH) are at baseline levels at this time. The uterine lining (endometrium) is shed creating the discharge of blood and tissue associated with menstruation. The average duration is 5 days however it can last longer or be shorter for some women. The low level of estrogen in the blood is recognized by the pituitary gland which then begins to raise FSH levels in response. The average length of menstrual phase is 5 days.
Proliferative or Follicular Phase
The next phase is the proliferative or follicular stage; the rising FSH levels triggers a regeneration of the endometrial tissue and approximately 20 Graafian follicles develop in the ovaries. As the follicles grow, estrogen begins a steady climb in the bloodstream. One follicle will outgrow the others and this one will be responsible for developing a mature egg (ovum). When the estrogen reaches high enough levels, the Follicle Stimulating Hormone is then suppressed; again, signals are sent to the pituitary gland, this time LH is produced. The average length of follicular phase is 10-14 days.
Secretory or Luteal Phase
Once the Graafian follicle containing the mature ovum ruptures (ovulation) it signals the third phase called the secretory or luteal phase. In this phase the ovum travels through the fallopian tube making its way to the uterus. At the same time this is occurring, fat globules begin attaching to the ruptured follicle creating a luteal cell and forms the corpus luteum. The altered follicle is now capable of producing progesterone which in turn stimulates the uterine endometrium preparing it for the egg to implant if fertilization is successful. If fertilization is not completed and implantation of the ovum does not occur, the corpus luteum atrophies and as it does the levels of progesterone fall. This then leads to the menstrual phase again. If the fertilized egg is implanted into the uterus, progesterone levels continue to rise to maintain a pregnancy. The average length of luteal phase is 14 days.
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