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Muscles and joints shift and jostle.

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

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

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Ignited by a range of hormones,

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these changes begin 
as soon as pregnancy begins.

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

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

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the immune system should theoretically 
recognize it as an invader,

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

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

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

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

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

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involving many different types 
of immune cells—

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

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

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which keeps germs away 
and stays sealed until labor.

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As a pregnancy progresses,

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the uterus expands upward and outward 
with the growing fetus.

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To make room, hormones 
called progesterone and relaxin

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signal muscles to loosen.

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

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which makes them sluggish,

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causing constipation as passage 
through the tract slows down.

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Loosened muscles 
at the top of the stomach

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might allow acid to escape 
into the esophagus and throat,

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causing heartburn and reflux.

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These changes can worsen morning sickness,

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

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As the uterus grows, 
it pushes on the diaphragm,

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

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This limits the diaphragm’s range.

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

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

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can both get enough oxygen 
with less lung capacity.

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

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Meanwhile, the kidneys 
make more erythropoietin,

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a hormone that increases 
red blood cell production.

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

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to build up the volume of the blood.

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

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But it’s also a bit diluted,

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because it only has 25% 
more red blood cells.

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

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

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from nutrients in the mother’s food—

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leaving less iron and other nutrients 
for the mother.

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

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through the body and placenta.

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A pregnant woman’s heart rate increases,

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

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an important area of research,

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because some of the most serious 
complications

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are related to the heart 
and blood pressure.

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The expanding uterus may press on veins—

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causing fluid buildup 
in the legs and feet.

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

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it might interfere with blood 
returning to the heart,

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

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Some of these changes start 
to reverse even before birth.

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Shortly before delivery, 
the fetus drops down,

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decreasing the pressure on the diaphragm

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and allowing the pregnant woman 
to take deeper breaths.

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During labor and birth,

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

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The uterus shrinks back down 
in the weeks after birth.

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

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but its effects here are some 
of the least understood.

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Recent studies show differences 
in brain scans

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after pregnancy and early parenting,

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and suggest that these changes 
are adaptive.

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That means they could help 
with parenting skills,

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

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

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

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has focused on the fetus, 
rather than pregnant women.

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Experiences of pregnancy vary widely,

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both within the range 
of healthy pregnancies

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

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and develop effective treatments 
where necessary.

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In the meantime, 
every pregnancy is different,

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

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Today, we’re turning an exciting corner,

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