The Menstrual Cycle (Menstruation)
Day 1 of your cycle is the first full day of menstrual bleeding.
The uterine lining built up in the previous cycle is cleared away, and the hormone
levels from the previous cycle take a sharp decline, causing the physical and emotional
symptoms commonly associated with menstruation.
Pre-Ovulation
The pituitary gland releases a hormone called FSH (Follicle Stimulating
Hormone), which stimulates an ovarian follicle to grow and an egg to mature. The pituitary
gland also releases LH (Luteinizing Hormone) which stimulates the follicle to manufacture
and secrete estrogen. Estrogen causes the uterine lining to grow.
Ovulation
The pituitary gland releases a heavy surge of LH. Home ovulation kits
are designed to detect this sharp increase. 24 to 36 hours after the surge, the follicle
will rupture, releasing the matured egg to the fallopian tube. The remainder of the
ruptured follicle (called the corpus luteum) recedes back to the ovary and begins to
secrete progesterone. Progesterone causes an increase in blood vessels to the uterine
lining, inhibits other eggs from developing, and causes the Basal Body Temperature
(BBT) to rise about half a degree.
Luteal Phase
The luteal phase is the period of time (usually 11-14 days) following
ovulation.
The egg can be fertilized within 24 hours of release, while it is still in the fallopian
tubes. If the egg is fertilized, the pituitary gland produces hCG which causes the increased
production of progesterone. The progesterone in turn causes the BBT to remain high throughout
the luteal phase and after the 14th day. High progesterone levels are also responsible for
"morning sickness" and other symptoms of pregnancy.
If the egg is not fertilized within 24 hours, the corpus luteum regresses and slows its
progesterone production. After around 11-14 days, there is no longer enough progesterone
to supply the uterine lining with blood, and so menstruation begins anew.
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