At puberty, a woman begins to ovulate. Ovulation is the release
of a mature fertilizable egg from the ovary. Ovulation should
continue throughout a woman's reproductive lifespan typically
at regular monthly intervals.
At each of the two ends of the reproductive lifespan, after the
onset of puberty and prior to menopause, menstrual cycle intervals
may be irregular and generally longer in duration.
A woman has an estimated 300 to 400 thousand eggs contained within
her ovaries at the time of puberty. She is usually presumed to
have no remaining eggs at menopause, although research has recently
demonstrated that she may actually have a few hundred eggs remaining
at menopause. Since a normal reproductive lifespan typically covers
the ages 14 to 44 there are about 30 years of ovulation with about
12 ovulations per year for a total of about 300 to 400 ovulations.
Thus, a woman "uses up" an average of about 1000 eggs
per menstrual cycle (300 to 400 thousand eggs in 300 to 400 ovulations).
It is not clear whether these eggs are used at a uniform rate
over the entire reproductive years or whether there is a certain
time period of rapid loss. It does appear that about 10-15 years
prior to menopause (at about the age 37-38 and continuing for
up to a few years) there is a period of accelerated loss of follicles
with an initial shortening of the egg maturing phase of the menstrual
cycle (follicular phase). This period of accelerated loss does
not fully account for the enormous loss of eggs over the reproductive
lifespan.
A follicle is a cyst in the ovary containing an egg. Most likely,
of the 1000 eggs that are committed for any given cycle only a
small number of follicles (maybe 5-25) actually begin the process
of maturation at the onset of the cycle.
Ovulation can be interrupted by physiologic events (such as during
pregnancy or breast feeding), medication (such as oral contraceptive
pills), or pathology (such as hormonal or anatomic abnormalities).
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