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Oral contraceptives are
medications that prevent pregnancy. They are
one type of birth control. Oral
contraceptives may contain
combinations of estrogen and progestin or
progestin alone. Combinations of estrogen
and progestin prevent pregnancy by
inhibiting the release of the hormones LH
and FSH from the pituitary gland in the
brain. LH and FSH play key roles in the
development of
the egg and preparation of the lining of the
uterus for implantation of the embryo.
Progestin also makes the uterine mucus that
surrounds
the egg more difficult for sperm to
penetrate and, therefore, for fertilization
to take place. In some women, progestin
inhibits ovulation (release of the egg).
The combination Oral contraceptives are
called "monophasic," "biphasic," or "triphasic."
Monophasic Oral contraceptives deliver the
same amount of
estrogen and progestin every day. Biphasic
Oral contraceptives deliver the same amount
of estrogen every day for the first 21 days
of the cycle.
During the first half of the cycle, the
progestin/estrogen ratio is lower to allow
the endometrium to thicken as it normally
does. During the second half o
f the cycle, the progestin/estrogen ratio is
higher to allow normal shedding of the
lining of the uterus to occur. The triphasic
Oral contraceptives have
constant or changing estrogen concentrations
and varying progestin concentrations
throughout the cycle. There is no evidence
that bi- or tri-phasic OCs
are superior to monophasic Oral
contraceptives, or vice-versa.
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