It all starts with a sperm and an egg.

Sperm are tiny microscopic cells

that are made in testicles.

Sperm cells mix with other fluids to make semen

which spurts out of your penis when you ejaculate.

Eggs live in ovaries.

And the hormones that control your menstrual cycle

cause eggs to mature every month.

These hormones also make the lining of you uterus

thick and spongy to prepare for a possible pregnancy.

Every month, one mature egg leaves your ovary.

This is called ovulation.

That egg travels through your fallopian tube

which takes about 12 to 24 hours.

If semens get into your vagina,

sperm can swim up through your cervix, uterus, and fallopian tubes,

searching for an egg.

Sperm cells can live in your body for up to six days,

waiting for an egg to show up.

Millions of sperms come out during each ejaculation,

but it only takes one sperm to meet with an egg

which can lead to pregnancy.

When a sperm cell joins with an egg,

it's called fertilization.

After that happens,

the fertilized egg begins to divide into more and more cells

and moves towards your uterus.

The ball of cells get to your uterus

about three to four days after fertilization,

where it can float around for another few days.

If the ball of cells attaches to the spongy uterine lining,

pregnancy officially begins.

This is called implantation.

It usually takes around three to four days to finish implanting.

Up to half of all fertilized egg naturally don't implant.

They passed out of your body during your period.

When the ball of cells implants into your uterine lining,

your body starts making pregnancy hormones.

These hormones keep your uterine lining in place

so it can nourish the pregnancy.

That's why you don't get your period

when you're pregnant.

But if sperm and egg don't meet up

or a fertilized egg doesn't implant into your uterus,

the lining isn't needed

and it flows out of your vagina.

That's your period.

So it actually takes a lot of steps

and up to two - three weeks

for pregnancy to happen.

Want to learn more about pregnancy?

Check out PlannedParenthood.org