Your menstrual cycle is driven by your hormones. Hormones are biochemical substances that are produced in one area of your body and carried in your bloodstream to send signals that trigger responses in another part of your body. The hormones that control your fertility signals are produced in the following areas:
hypothalamus (in your
gland (also in your brain, located behind and attached to the
(located on top of your kidneys).
At the beginning of
your menstrual cycle, the hypothalamus produces GnRH
(gonadotropin-releasing hormone). The GnRH pulses through your
bloodstream from the hypothalamus to the pituitary gland in spurts
every 60-90 minutes from menstruation until ovulation. The GnRH
signals the anterior pituitary gland to secrete FSH (Follicle
Stimulating Hormone) and later LH (Luteinizing Hormone). This is
what is happening when you have your period and in the days before
Stimulating Hormone (FSH), as its name suggests, stimulates the
development and maturation of follicles in the ovaries. One of these
follicles will become dominant and contains the ovum that will be
released at ovulation. The developing follicles begin to produce
The estrogen released
by the developing follicles, and later by the dominant follicle,
causes the lining of the uterus, the endometrium, to grow and
thicken in preparation of implantation of a fertilized ovum.
By about the seventh
day of your cycle on average (but this can vary widely) the dominant
follicle takes over. The eggs within the other follicles lose their
nourishment and die as do the follicular cells.
The dominant follicle
produces a sharp rise in estrogen. (You can recognize this stage of
your cycle by closely monitoring your cervical fluid). Estrogen is
at its peak one to two days prior to ovulation.
This estrogen surge
signals the release of LH (Luteinizing Hormone). This is the hormone
that is measured by ovulation predictor kits (OPKs). LH travels
through the bloodstream to the ovary where it causes the ovary to
release enzymes that make a hole in the sac of the dominant
follicle. This causes the dominant follicle to rupture and release
the ovum into the fallopian tube where it can be fertilized. This is
ovulation. The LH surge is necessary for ovulation to occur. The LH
surge (the highest concentration of LH) occurs 12-24 hours prior to
ovulation but LH begins to rise about 36 hours before ovulation.
dramatically after ovulation.
follicle, transformed by LH, becomes the corpus luteum after
ovulation. This phase of your cycle after ovulation is called the
luteal phase since hormone production is governed by the corpus
luteum. The corpus luteum continues to produce a small amount
estrogen, but now also produces progesterone. Progesterone is the
hormone that dominates this phase of your cycle. Like estrogen,
progesterone is needed to develop the endometrium so that a
fertilized egg can implant and be nourished should fertilization
occur. Your BBT (Basal Body Temperature) rises as a result of
If an egg is
fertilized and implantation of the fertilized egg occurs, the corpus
luteum's life is extended. In conception cycles, the corpus luteum
keeps on producing progesterone and some estrogen and the
development of the endometrium continues. The pregnancy hormone, hCG
begins to be produced when the fertilized egg implants, at around
7-10 days past ovulation. As the pregnancy progresses, hormone
production is taken over by the placenta.
If there is no
pregnancy, the corpus luteum dies, progesterone levels fall, and a
new cycle begins.
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