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Conception and
pregnancy are governed by the ebb and flow of sex hormones
that each month prompt crucial changes in your reproductive
system. Production of these hormones depends, in turn, on the
changes they themselves produce, so that an elegant cycle of
feedback and response dictates their
levels.
The most notable
outward sign of this carefully balanced interplay is, of
course, your monthly menstrual cycle or period. This cycle
begins with your first day of menstrual bleeding and ends at
the start of the next period. The average cycle is from 25 to
34 days and the average menstrual flow lasts from 3 to 5
days
The menstrual cycle
has two distinct phases: the follicular (proliferative) phase
during which the egg grows and gets ready to enter the
fallopian tube; and the luteal (secretory) phase when the
corpus luteum is prepared to help maintain a possible
pregnancy. The endometrium, or uterine lining, starts to
grow, and reaches its greatest thickness during the luteal
phase. If conception fails to occur, the lining is then
discarded in the menstrual flow, and the cycle begins anew.
This entire circle of changes is directed by the on/off
production of six key hormones.
While many hormones
interact in the reproductive process, perhaps the three most
well-known are estrogen, progesterone, and
androgen.
There are several
forms of estrogen but the one most important for reproduction
is estradiol, a substance secreted by the ovary. In addition
to being responsible for the development of sexual
characteristics in women, estrogen governs the monthly
thickening of the endometrium and the quantity and quality of
cervical and vaginal mucus so important to the successful
passage of the sperm.
Progesterone, the
principle hormone secreted by the corpus luteum, is chiefly
responsible for preparing the endometrium to accept a
fertilized egg. The corpus luteum continues to secrete
progesterone during the first three months of pregnancy until
the placenta can fend for itself.
Androgen is
produced by follicle cells in the ovary and is converted into
additional estrogen. Androgen causes the disappearance of all
of the follicles not destined to produce an egg during a
given monthly cycle.
Gonadotropin
Releasing Hormone
Called
“GnRH” for short, this is the hormone that governs
the level of estrogen in your body. It is produced by the
hypothalamus, a gland located at the base of the
brain.
At the end of your
mentstrual cycle, declining levels of estrogen in your
bloodstream spark the hypothalamus into a burst of activity,
doubling or even tripling productin of GnRH. Production
occurs in pulses. During the first, or follicular, phase of
your cycle, when production is highest, the pulses come at
hourly intervals. Later, during the luteal phase, they slack
off to about once every two or three hours. Finally, as the
luteal phase ends and estrogen levels reach their lowest ebb,
the cycle starts again.
Gonadotropins
GnRH does its work
through two intermediaries: follicle stimulating hormone (FSH
for short) and luteinizing hormone (LH for short). These two
hormones, known as gonadotropins, are produced by the
pituitary gland. When levels of GnRH rise in your
bloodstream, the pituitary responds by increasing its release
of FSH and LH. The two hormones are then free to begin
working changes in the ultimate target, the ovary and the
egg-containing follicles.
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KEY PLAYERS IN THE MONTHLY HORMONAL
CYCLE
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Two master
hormones govern all the others.
Gonadotropin Releasing Hormone (GnRH) from the
hypothalamus in the brain sparks release of follicle
stimulating hormone and luteinizing hormone from the
pituitary gland, which in turn prompt production of
estrogen and progesterone in the ovaries. If conception
occurs,
Human Chorionic Gonadotropin (HCG) from the
developing placenta takes over, perpetuating production
of progesterone. High progesterone levels shut down
production of GnRH, leaving HCG in control for the
duration of the pregnancy.
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Next:
The Menstrual
Phases
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