Phases Of The Menstrual Cycle
The Menstrual Cycle
The menstrual cycle, common to females of reproductive age, is a cycle of hormonal activity that repeats every 28 to 35 days. Every cycle sees a preparation for a potential pregnancy with the lining of the uterus thickening in order to receive a fertilized egg. If there is no conception, then the lining of the uterus is shed through the cervix and it passes out of the body through the vagina.
There are four major hormones involved in the menstrual cycle: follicle-stimulating hormone, luteinizing hormone, estrogen, and progesterone. These hormones affect the phases of the menstrual cycle: the follicular phase, the ovulatory phase, and the luteal phase.
The Follicular Phase
The follicular phase of the menstrual cycle begins on the first day of menstruation. During this phase the brain releases two hormones, follicle stimulating hormone (FSH) and luteinizing hormone (LH), and these hormones travel via the blood to the ovaries and stimulate the growth of 15 to 20 eggs in the ovaries. Each egg is in its own shell, called a follicle. Estrogen production is also increased with the release of LH and FSH. As levels of estrogen rise, the production of FSH is shut off allowing the body to limit the number of follicles that will mature. During the follicular phase, one egg in one ovary dominates and grows. The other follicles in the group die and the dominant follicle continues to produce estrogen.
The Ovulatory Phase
About two weeks after the follicular phase starts, the ovulatory phase (ovulation) begins at what is the midpoint of the cycle. The next menstruation will begin about two weeks after ovulation. During the ovulatory phase there is a surge of luteinizing hormone that is produced by the brain. This surge is triggered by the rise in estrogen from the dominant follicle and the egg is then released from the ovary. The egg is caught in the finger like projections that extend down from the ends of the fallopian tubes (fimbriae) and is swept into the tube. The cervix produces mucous and at this time the production of mucous picks up and the amount increases and is thicker. This thicker mucous captures a man's sperm and nourishes it, moving it toward the uterus and into the fallopian tube where it will fertilize the egg.
The Luteal Phase
The luteal phase of the menstrual cycle begins immediately after ovulation and during this phase the follicle that released the egg is now empty and develops into the corpus luteum which secretes progesterone. This hormone prepares the uterus for the implantation of the egg. If fertilization of the egg has taken place (conception) then the fertilized egg will travel through the fallopian tube into the uterus. There it will implant into the wall of the uterus and pregnancy occurs. If the egg is not fertilized, it passes through the uterus and since the lining of the uterus is not needed to house a fertilized egg, it breaks down and is sloughed off. This is the beginning of the next menstruation.