A baby lives
upside down.
Why no
head rush?

The full narration of the video.

Stand on your head for a minute, and your face turns bright red and your head starts to throb. But a baby in the womb stays head-down for a month or two. Why doesn't that bother it? First, is it even true? Inside the belly, a baby tumbles around for most of pregnancy.

Around the eighth month, most babies turn head-down and stay that way until birth. By the due date, only three or four in a hundred are still bottom-down. Why head-down? The usual answer: the head is the heaviest part, and the womb is wider at the top, so the bottom and legs fit best up there. So why doesn't its face go red? Let's go through the guesses one by one.

Guess one: the baby isn't upside down all the time. When mom lies down to sleep, the baby lies sideways too. True, but during the day, whenever mom stands or sits, it's head-down again. Guess two: the baby is small. When an adult flips over, the head sits more than thirty centimeters below the heart. The blood presses down hard.

In a baby, heart to head is only ten-something centimeters, so the push is much gentler. Also true, but not the whole story. The real secret is in the bag of water around the baby. Try a little experiment. Hold up a balloon full of water. The water sags, and the bottom bulges into a big belly.

Now push the same balloon all the way into a tub of water. It stays round. No more sagging. Why? Water pushes harder the deeper you go. The water inside pushes out, the water outside pushes in, and both grow together on the way down, so they cancel out. The baby is that balloon in the tub. It floats in the fluid of the womb, which is almost all water, and blood weighs about the same as water.

Down at the head, the blood pushes out harder, but the fluid outside pushes in harder too. They balance, and the blood vessels don't stretch. So blood doesn't pile up in the head. An adult upside down in the air has nothing pushing back from outside. So the tiny vessels in the face stretch open, and you go red and throbbing. Doctors who study diving have measured this. In water, the water pressure around your ankle roughly matches the push of the blood column in your leg.

Still, nobody can climb into a womb to measure a baby's head. This part is worked out from physics. So it's not that babies don't mind being upside down. A whole bag of water is holding them up from every side. Parents, how many weeks along was your baby when it turned head-down? Drop the number in the comments.