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As estrogen and
progesterone levels bottom out at the end of the menstrual
cycle and a new one begins, the hypothalamus hikes production
of GnRH, switching the pituitary over to higher output of FSH
and LH. FSH levels are at their peak at this point, allowing
this hormone to successfully stimulate about 20 egg follicles
in one of the ovaries.
Within each of
these follicles, FSH fosters an increase in the number of
cells that produce estradiol, the most important kind of
estrogen. Meanwhile, LH is at work on a different set of
cells within the follicle, encouraging increased production
of androgen. The balance between the relative amounts of
estradiol and androgen produced within each follicle
determines its survival.
One
follicle—the one that produces the most
estradiol—eventually predominates. The more estradiol
this follicle produces, the more FSH it attracts. But the
estradiol it makes simultaneously signals the pituitary gland
to cut back on its secretion of additional FSH. This, in
effect, starves the other follicles for FSH and they begin to
disappear, as the dominant follicle attracts more and more of
the available FSH and makes more and more
estradiol.
Eventually, the egg
within this follicle develops the gelatinous coat described
earlier and gets ready to leave the ovary and enter the
fallopian tube. Meanwhile, the increasing levels of
estradiol, along with a rise in progesterone and declining
levels of FSH, prompt the pituitary to release a surge of
LH.
When the egg is
ready for release, roughly the 14th day of the normal
menstrual cycle, LH levels peak. Release (ovulation) occurs,
and the remnants of the dominant follicle remain behind,
awaiting the next phase of the cycle, when they will be
transformed into the corpus luteum, the “yellow
body” that supports the developing baby, during the
first three months of life.
Now progesterone
becomes the key hormonal player. The ruptured follicle, the
first portion of its reproductive role completed, develops
its own blood supply. It then starts to produce progesterone
that simultaneously halts any further follicular growth and
governs the preparation of the endometrium which has already
been thickened in the first phase of the cycle. The corpus
luteum also produces estrogen to help prepare the endometrium
for potential pregnancy.
About a week after
you ovulate, your progesterone levels peak, then begin to
decline along with your estrogen levels. If you have
conceived a child, the developing placenta will produce yet
another hormone called human chorionic gonadotropin (HCG).
This hormone will maintain the corpus luteum, which will
continue to produce progesterone to help maintain the
pregnancy. On the other hand, if you are not pregnant, there
will be no HCG, your LH levels will fall, and your corpus
luteum will rapidly dry up and turn into a scar tissue. Low
estrogen and progesterone levels will once again prompt the
hypothalamus to speed production of GnRH, and your
reproductive system will swing into action again.
Whether the goal is
to increase your chances of conception or reduce the odds of
pregnancy, many of the techniques described in the following
chapters do their work by adjusting hormone levels at various
points in this cycle. Birth control pills, shots, and
implants all rely on this strategy, as do many fertility
treatments. The understanding of this cycle by scientists has
truly revolutionized our reproductive options.
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FROM MENARCHE TO MENOPAUSE
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While we tend
to think that the female reproductive system remains
dormant until a girl is somewhere between 10 and 14, her
sex hormones actually begin to function in the first few
months after conception. In fact, GnRH, FSH and LH start
to circulate in the fetus shortly after the ovaries
develop. Surprisingly, the levels of FSH and LH in the
developing baby are very similar to those found in
postmenopausal women!
After a rather
turbulent first year during which the newborn baby's
system makes the final transition from reliance on her
mother's hormones to her own, the reproductive system
enters a dormant phase. FSH and LH levels fall to their
lowest levels by the time girls are about two years old
and then start to rise slowly again between the ages of
four and ten.
Starting as
early as age six, circulating levels of the male hormone,
androgen, start to increase. This rise in androgen
eventually causes the development of hair under the arms
and in the pubic area. Meanwhile, estrogen stimulates
breast development, and the sex hormones and glands begin
to gear up for menarche, the onset of menses.
Even in an era
when so many of the body's secrets are being unlocked,
scientists still are not sure exactly what sets off the
transformations of puberty.
However, at
some point between the ages of 8 and 14 something causes
pulses of LH to be secreted at a rate 2 to 4 times higher
at night than during the day. This prompts estrogen and
FSH levels to rise, and puberty is underway.
Puberty is
traditionally defined by three events: development of the
breasts, appearance of pubic hair, and the beginning of
menstruation. It is important to remember that the time
it takes for these changes to occur is extremely
variable. While one girl may appear to be fully developed
by age 11, her friend next door may not reach puberty
until age 14 or 15. Both schedules are considered
“normal.”
After puberty,
the reproductive system continues its regular cycles
until around age 40, after which the perimenopausal years
begin. At this point, ovarian function and the monthly
menstrual cycle tend to become less regular, and many of
the effects of estrogen in the body start to wane.
Menopause (the cessation of the monthly cycle and the end
of reproductive capacity) usually occurs between the ages
of 45 and 55 in American women; the average age is about
51. For more on this stage in a woman's life, see the
section on menopause beginning on page 351.
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