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Here's what has to happen for pregnancy
to occur after sexual intercourse.

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Sperm must swim up the vagina,

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through the cervical opening,

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upwards through the uterus,

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and into one of the two fallopian tubes.

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If an egg, released during 
that month's ovulation, is in the tube,

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one sperm has a chance to fertilize it.

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Contraceptives are designed
to prevent this process,

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and they work in three basic ways.

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They block the sperm,

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disable sperm 
before they reach the uterus,

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or suppress ovulation.

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Block is the simplest.

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Male and female condoms prevent
sperm from coming into contact

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with the vaginal space.

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That barrier is also why they,
unlike other contraceptive methods,

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are able to prevent transmission
of certain sexually transmitted diseases.

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Meanwhile, the diaphragm, 
cervical cap, and sponge

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work by being placed over the cervix,
barricading the entrance to the uterus.

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These contraceptives are sometimes
called barrier methods

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and can be used with spermicides,

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an example of 
the second category, disable.

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A spermicide is a chemical 
that immobilizes and destroys sperm.

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Today's spermicides come as foam,
cream, jelly, suppositories,

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and even a thin piece of translucent film
that dissolves in the vagina.

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These products can be inserted directly
into the vagina before intercourse,

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or can be combined with block methods,
like a diaphragm or condom,

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for added proection.

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The third category 
for preventing pregnancy

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works by suppressing the action of an egg
maturing in the ovary.

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If there isn't an egg available 
in the fallopian tube,

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there's nothing for sperm to fertilize.

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Hormonal contraceptives,

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including the pill, the patch, 
the Depo shot, and the vaginal ring

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all release synthetic versions of various
combinations of progesterone and estrogen.

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This hormone cocktail 
suppresses ovulation,

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keeping the immature age 
safely sequestered in the ovary.

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Synthetic progesterone also has a block
trick up its sleeve.

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It makes cervical mucus too thick and
sticky for sperm to swim through easily.

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There are other contraceptives that
use multiple approaches at the same time.

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For example, many IUDs,
or intrauterine devices,

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contain synthetic hormones 
which suppress ovulation.

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Some also contain copper,

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which disable sperm while also making
egg implantation in the uterus difficult.

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Block, disable, or suppress.

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Is one strategy better than the other?

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There are differences, but a lot of it
has to do with how convenient

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and easy it is to use 
each contraceptive correctly.

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For example, male condoms would be
about 98% effective

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if everyone used them perfectly.

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That 98% means if 100 couples correctly
used condoms for a year,

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two women would get pregnant.

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But not everyone uses them correctly,
so they're only 82% effective in practice.

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Other methods, like the patch and pill,
are 99% effective

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when they're used perfectly.

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But in practice, that's 91%.

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Spermicide is only 85% effective,
even with perfect usage,

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and just 71% effective with typical usage.

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Another important consideration in the
choice of contraceptives are side effects,

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which almost exclusively effect women
rather than men.

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Hormonal methods in particular
can cause symptoms like headaches,

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nausea,

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and high blood pressure,

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but they vary from woman to woman.

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That's why these methods
require a prescription from a doctor.

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The choice of contraceptive method
is a personal one,

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and what works best for you now
may change later.

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Scientists also continue to research
new methods,

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such as a male pill that would
prevent sperm production.

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In the meantime, 
there are quite a few options

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to block sperm,

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disable them,

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or suppress eggs
and keep them out of reach.