Lungs full
of water: how
do babies
breathe?
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People say a baby in the womb has lungs full of amniotic fluid. That's only half right. The lungs really are full of liquid. Yet when we choke on one tiny sip of water, it's awful. So how does a baby soak in it for nine months and not suffocate? First guess: does the baby pull oxygen out of the water, like a little fish?
No. Babies have no gills, and their lungs can't grab oxygen from water. Second guess: is the baby just holding its breath the whole time? Not that either. On an ultrasound, the little chest often rises and falls, like it's practicing breathing. In the last months before birth, one study counted: over a third of the time, the baby is practicing.
But it's only practice. In and out goes nothing but liquid, and not a bit of oxygen comes with it. So where does the oxygen come from? The clue is right at the belly button. Mom takes a breath, and oxygen enters her blood. Her blood flows to a soft, round cake on the wall of the womb, called the placenta.
Inside the placenta, mom's blood and baby's blood sit very close, with a thin wall between them, so they never mix. Oxygen slips through, from mom's side to the baby's. Baby blood is extra good at grabbing oxygen, holding on tighter than mom's. So as soon as oxygen crosses over, the baby snatches it. Oxygen-rich blood travels down the umbilical cord into the baby's body. The used-up waste gas rides the cord back, and mom breathes it out for the baby.
So it's mom who breathes for the baby. Every breath she takes, she shares a little. Then where does the liquid in the lungs come from? Mostly not amniotic fluid: the lungs make it themselves, bit by bit. It keeps the lungs stretched open so they can grow. Any extra slowly flows out of the mouth and joins the amniotic fluid. The baby's heart also has two shortcuts open, so blood can skip the lungs it can't use yet.
The big moment comes at birth. During birth, the baby gets squeezed, and some liquid runs out of the mouth and nose. The rest is soaked back into the body. It's usually all gone within a day. The first cry is the first breath of air. The lungs pop open. The shortcuts in the heart start to close, and blood begins to flow into the lungs.
From that moment on, the baby stops relying on mom and breathes on its own. So what did you use to think: how did babies breathe in the womb? Tell us in the comments.
"Full of amniotic fluid"?
Only half right
On the ultrasound, the baby's chest sometimes rises and falls, as if breathing. But the baby is floating in water, and its lungs are full of water too.
The lungs are full of fluid,
but mostly not amniotic fluid —
and the baby doesn't breathe with them.
Nine months underwater without drowning. So where does the oxygen come from?
Like a fish?
Holding its breath?
Neither
The clue isn't in the chest. It's at the belly button.
Mom's blood carries
oxygen to the placenta
With every breath,
mom shares a little.
The baby's blood
grips tighter
The stuff in a baby's blood that carries oxygen is a little different from an adult's, and it holds on tighter. So across that thin layer, oxygen naturally drifts toward the baby.
The lungs make
their own fluid
Most of that fluid isn't amniotic fluid that leaked in. The lungs make it themselves, bit by bit. It keeps the little lungs stretched open and helps them grow; the extra flows out of the mouth into the amniotic fluid.
When the baby practices breathing, a tiny bit of amniotic fluid may get in too.
Two shortcuts
in the heart
Since the lungs aren't in use yet, blood doesn't need to go through them. A baby's heart has two shortcuts: a small hole between its upper chambers, and a little tube straight to the main artery. Most blood skips the lungs.
The lungs rest,
waiting for birth.
The first breath
In the womb, babies also hear mom talking, kick around, and near the end turn head‑down.
mom breathes
for the baby.
babies breathed in the womb?
Quick check
Where does a baby in the womb get its oxygen?
The lungs are full of fluid and don't trade gas; every breath mom takes, a little oxygen goes via the placenta and down the cord.
