Muscles and joints shift and jostle.

The heart’s pounding rhythm speeds up. 
Blood roars through arteries and veins.

Over the course of a pregnancy, 
every organ in the body changes.

Ignited by a range of hormones,

these changes begin 
as soon as pregnancy begins.

Just days after fertilization, the embryo 
implants in the lining of the uterus.

Because its DNA doesn’t exactly match 
the mother’s,

the immune system should theoretically 
recognize it as an invader,

attack, and destroy it, like it would 
bacteria or other harmful microbes.

That’s the challenge: 
the mother’s immune system

needs to protect both her and the fetus, 
but can’t act as it usually does.

What happens is not as simple 
as decreasing the immune response.

Instead, it’s a complex interaction 
we’re just beginning to understand,

involving many different types 
of immune cells—

some of which seem to protect the fetus 
from attack by other immune cells.

The body also creates an antibacterial 
plug made of mucus on the cervix,

which keeps germs away 
and stays sealed until labor.

As a pregnancy progresses,

the uterus expands upward and outward 
with the growing fetus.

To make room, hormones 
called progesterone and relaxin

signal muscles to loosen.

The muscles that propel food and waste 
through the digestive tract also loosen,

which makes them sluggish,

causing constipation as passage 
through the tract slows down.

Loosened muscles 
at the top of the stomach

might allow acid to escape 
into the esophagus and throat,

causing heartburn and reflux.

These changes can worsen morning sickness,

which is caused in part by hormone HCG— 
and can also happen at other times of day.

As the uterus grows, 
it pushes on the diaphragm,

the muscle that expands and contracts 
the chest with each breath.

This limits the diaphragm’s range.

To compensate, the hormone progesterone 
acts as a respiratory stimulant,

making the pregnant woman breathe 
faster so both she and the baby

can both get enough oxygen 
with less lung capacity.

This all may leave the pregnant woman 
feeling short of breath.

Meanwhile, the kidneys 
make more erythropoietin,

a hormone that increases 
red blood cell production.

The kidneys also keep extra water and salt
rather than filtering it out into urine

to build up the volume of the blood.

A pregnant woman’s blood volume 
increases by 50% or more.

But it’s also a bit diluted,

because it only has 25% 
more red blood cells.

Usually, the body makes blood cells 
using iron from our food.

But during pregnancy, the fetus 
is also building its own blood supply

from nutrients in the mother’s food—

leaving less iron and other nutrients 
for the mother.

The heart has to work extra hard 
to pump all this blood

through the body and placenta.

A pregnant woman’s heart rate increases,

but we don’t fully understand how blood 
pressure changes in a healthy pregnancy—

an important area of research,

because some of the most serious 
complications

are related to the heart 
and blood pressure.

The expanding uterus may press on veins—

causing fluid buildup 
in the legs and feet.

If it presses on a large vein 
called the inferior vena cava,

it might interfere with blood 
returning to the heart,

causing a dizzying drop in blood pressure 
after standing for too long.

Some of these changes start 
to reverse even before birth.

Shortly before delivery, 
the fetus drops down,

decreasing the pressure on the diaphragm

and allowing the pregnant woman 
to take deeper breaths.

During labor and birth,

much of the extra fluid in the body 
is lost when the water breaks.

The uterus shrinks back down 
in the weeks after birth.

Like the rest of the body, 
pregnancy affects the brain—

but its effects here are some 
of the least understood.

Recent studies show differences 
in brain scans

after pregnancy and early parenting,

and suggest that these changes 
are adaptive.

That means they could help 
with parenting skills,

such as an increased ability to read 
facial cues since babies can’t talk.

The lack of information about pregnancy’s 
effects on the brain

highlights a general truth: historically, 
almost all the research around pregnancy

has focused on the fetus, 
rather than pregnant women.

Experiences of pregnancy vary widely,

both within the range 
of healthy pregnancies

and due to complicating health conditions—
new research will help us understand why,

and develop effective treatments 
where necessary.

In the meantime, 
every pregnancy is different,

and it’s important to consult a doctor 
with any specific questions.

Today, we’re turning an exciting corner,

as more research is devoted 
to the astounding biology of pregnancy.