The surprising effects of pregnancy
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.