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Pregnancy 101 | National Geographic

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- [Narrator] The product of millions of years of evolution,

the human body is capable of many remarkable things,

but none of which may be quite so incredible

as the development of life in utero.

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Over three million babies are born

each year in the United States alone.

Worldwide, the highest fertility rate is found

in Niger, where the average woman gives birth

to approximately 6.49 children in her lifetime.

Singapore sits on the opposite end of the spectrum

at just 0.83, less than one birth per woman.

While the number of births, customs,

and traditions vary from culture to culture,

the developmental process is essentially universal.

Derived from the Latin word praegnantem,

meaning before birth, pregnancy is the period

in which the fetus develops inside the womb.

Typically lasting around 40 weeks,

human pregnancies are divided

into three trimesters of three months each.

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Pregnancy begins in the uterus,

where a sperm fertilizes an egg.

If the sperm carries an X chromosome,

the baby will become female,

while a Y chromosome will result

in the baby becoming male.

The fertilized egg, or zygote,

divides repeatedly as it travels

through the fallopian tube,

implanting itself on the uterine wall

to form both the embryo

and a specialized organ known as the placenta.

Found only in eutherian, or placental, mammals,

the placenta will manage waste and provide key nutrients,

oxygen, and hormones via the umbilical cord.

The brain, which will continue

to grow and develop throughout the pregnancy,

makes up nearly half of the embryo in these early stages.

As the eyes, nose, ears, and mouth,

along with all major organs continue

to develop in month three,

the baby will begin to look more

and more human with each passing day.

The second trimester lasts from weeks 13 through 27.

The fetus will more than double

in size during this time, and soon,

its movements may be felt by the mother.

Hearing first develops around week 18,

but the fetus will not respond to sounds

outside of the womb until approximately week 25.

Starting at week 28 and lasting up until delivery,

the third trimester is a time for final touches,

such as eyelashes and taste buds.

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With most major development complete,

the fetus will gain nearly half a pound a week.

To make room for this rapid growth,

the mother's internal organs adjust significantly

throughout the pregnancy.

Dropping lower into the pelvis,

a fetus typically turns heads-down

in preparation for birth.

Most bones will have hardened by this time,

though the skull will remain relatively soft

to ease the delivery process.

Labor is divided into stages,

beginning with the dilation of the cervix

and resulting in the delivery

of both the baby and the placenta.

Despite thousands of years of human pregnancies,

scientific understanding has only recently begun

to catch up, leading to an increase in success

and safety for both mother and child.

As our understanding of pregnancy continues to develop,

so do technology and reproductive medicine,

with much more in store for the future of pregnancy.