The Menstrual Cycle and
the Role of Hormones
Puberty occurs at various ages but, on average, females are around 12 when
they experience their first menstrual period and become capable of
reproduction. While still in the womb, the female develops some seven million
eggs; however, this ovarian
reserve has already dropped to approximately one million eggs at birth. By
the age of her first period, about 300,000 to 400,000 eggs remain. Typically,
only one healthy egg matures in the ovaries each month, before release into
the fallopian tube in preparation for fertilization. This process continues
for several decades and ends with menopause, which is triggered by depletion
of the ovarian reserve.
Although menopause occurs around age 51 on average, it is influenced by many
factors. These include a woman’s age at her first period, her mother’s
history and her ovarian reserve. Before menopause, most women experience
menstrual periods approximately every 28 days. The result is 10 to 11 periods
each year, unless a woman is taking "the pill" for birth control, is
pregnant or has clinical problems affecting her reproductive system. At age
forty, even though a woman may experience regular menses, her chances of
pregnancy diminish significantly due largely to the quality of eggs remaining.
In fact, at age 19, the likelihood that an egg will be fertilized during
unprotected sex is approximately 26%. By age 41, the chances are fewer than
5%.
Significant variation occurs amongst women when it comes to the length of
their cycles - the time between the first day of bleeding during one menstrual
period and the start of the next. Although the average is 28 days, a
"normal" cycle can be as few as 21 days or as many as 35. For most
women, bleeding occurs for approximately five days, but even normal periods
can last anywhere from three to six days.
The bleeding is caused by the shedding of the lining of the uterus, in cases
where an egg was not fertilized during a cycle. Had pregnancy occurred, the
uterine lining would have been ready to accept and nurture the fertilized egg
(embryo)
and, in most cases, the female’s menstrual cycle would stop during gestation
and begin again following delivery.
The development of an egg takes approximately 90 days, with maturation
occurring during menstrual bleeding, when it is still protected by a
fluid-filled sac (follicle)
within the ovary. A chemical called follicle
stimulating hormone (FSH), produced by the pituitary gland, aides this
growth process. During this part of the female’s cycle, two other
reproductive hormones - estrogen and progesterone - are at their lowest
levels.
The levels of estrogen and androgen secreted by the ovaries will rise as the
egg matures, from about the sixth to the fourteenth day of the cycle. Androgen
may increase the woman’s desire for sexual intercourse, while estrogen
causes the uterine lining to grow and rebuild from the previous cycle,
preparing it to accept an embryo.
By about the fourteenth day, the estrogen level is high enough to trigger a
surge of luteinizing
hormone (LH), another chemical produced by the pituitary. Some 34 to 36
hours after the surge begins – and about 10 to 12 hours after it peaks - ovulation
occurs. During ovulation, the egg is released from the follicle and into the
fallopian tube, where is can be fertilized by the sperm.
After ovulation, the follicle becomes the corpus
luteum and continues to produce androgen, estrogen and progesterone. The
progesterone works with the estrogen to change the uterine lining and the
mucus that lines the cervix. In some cases, the mucus becomes toxic to sperm,
affecting its ability to reach and fertilize the egg.
The window of opportunity for fertilization is limited to the time around
release of the egg into the fallopian tube for its journey to the uterus.
Whether pregnancy occurs depends on many factors including the availability of
a healthy egg and an adequate number of high quality sperm. In addition to
being plentiful, the sperm must be strong enough to swim through the vagina,
the mucus in the cervical canal and the uterus, before entering the fallopian
tube. Once it reaches the egg, it also must penetrate the outer shell (or zona
pellucida), which is likely to be hard and impenetrable in older women.
The fertilized egg must then travel and implant in the prepared uterine lining
in order to develop.
If fertilization does not occur, the corpus luteum breaks apart and levels of
estrogen and progesterone drop. Without these hormones to support it, the
uterus sheds its lining and menstruation begins about 13 to 15 days after the
peak in the LH surge.
