A series of events called the menstrual cycle happens about once every month
to prepare a woman’s body for pregnancy.
Changing levels of natural chemicals in the bloodstream called
called hormones
control these events.
The reproductive organs affected by these hormones include:
the vagina,
cervix,
uterus,
fallopian tubes,
and ovaries.
The ovaries produce two main hormones
called estrogen
and progesterone.
As the level of estrogen begins to rise,
it causes the normally thick mucus inside the cervix to thin out.
Estrogen also triggers other hormones to cause one of the ovaries to release an egg.
This process is called ovulation.
If a woman has sex during this time, a man’s reproductive cells,
called sperm,
can pass through the thinner mucus to fertilize an egg.
In the uterus,
estrogen causes the lining to thicken,
which prepares it to receive a fertilized egg.
Rising progesterone levels
cause glands in the lining to release fluid that feeds the fertilized egg.
Progesterone also causes the thinned out mucus in the cervix to become thick again,
which prevents sperm from passing through.
If an egg hasn’t been fertilized, the levels of both estrogen and progesterone begin to fall.
This drop in hormone levels causes menstruation,
a process where the uterus sheds its inner tissue lining and blood through the vagina.
Birth control pills are medications that a woman takes every day to prevent pregnancy.
The most common and effective type of birth control pill is a combination pill.
It contains both estrogen and progestin, which is a man-made hormone similar to progesterone.
The estrogen and progestin from the pill maintain constant levels of these hormones in the body.
These levels prevent the body from releasing similar hormones that cause ovulation.
Without ovulation, there is no egg available for fertilization, so a woman can’t get pregnant.
Progestin also prevents pregnancy by keeping the mucus in the cervix thick enough, so sperm can’t get through it.
A third effect of progestin on pregnancy involves its influence on the lining of the uterus.
In contrast to natural progesterone, progestin is slightly different chemically.
Over time, it makes the uterine lining thinner instead of thicker.
As a result, if fertilization of an egg does take place, the lining may be too thin
for it to stay in the uterus, so it passes out of the body with the next menstrual period.
Combination pills usually come in 28-day packs.
Depending on the brand of pill, the first 21 to 24 pills are active pills
because they contain hormones.
The last four to seven pills are inactive or “reminder” pills that don’t contain any hormones.
Inactive pills allow the hormone levels in the body to drop so that bleeding during the menstrual cycle can occur.
Even though the inactive pills don’t contain any hormones,
the woman remains protected against pregnancy during this time.
Inactive pills are also called “reminder” pills
because they keep a woman in the habit of taking a birth control pill every day.
Progestin-only pills or “mini pills” only contain progestin.
Like the combination pill, the progestin in the progestin-only pills prevents pregnancy by preventing ovulation,
thickening cervical mucus,
and thinning out the lining of the uterus.
Women may choose progestin-only pills if they can’t tolerate the estrogen in combination pills due to side effects or other medical reasons.
All progestin-only pills are active.
Menstrual bleeding with progestin-only pills may occur once every month,
on and off throughout the month, or there may be no bleeding at all.
For the highest protection against pregnancy, it is important to take progestin-only pills at the same time every day.
Both combination and progestin-only birth control pills
are about 99% effective at preventing pregnancy if used correctly.
This means about one out of 100 women will become pregnant each year if they use them correctly every day.
However, they are only about 92% effective with typical use,
which means about eight out of 100 women will become pregnant each year if they don’t always remember to take them every day.