Why does pregnancy
make you sick?

The full narration of the video.

A pregnant dog keeps eating. A pregnant woman smells dinner and feels sick. So what's the difference? Up to seven in ten pregnant women feel sick. It's called morning sickness, but it can come day or night. For most, it fades by month four or five.

To find the cause, follow a letter. It's sent from the baby's side. The baby feeds from the mother's blood through a soft pad. That pad is the placenta. The placenta releases a signal into her blood whose job is to make her feel sick.

In 2023, a study led by Cambridge found that nearly all of this signal is made by the baby's side. It drifts through her blood, all the way to the bottom of her brain. Down there is a small spot in charge of throwing up. It has no shield, so things in the blood can touch it directly.

It has a mailbox that takes only this letter. Letter goes in, alarm goes off: you ate something bad, get it out! But that doesn't explain something odd: some women are too sick to get out of bed, while others barely notice. The clue is in a rare blood disorder. People with it have high levels of this signal even before pregnancy. And when they get pregnant, they hardly feel sick.

And women with very low levels before pregnancy tend to be sicker. It's like living by the railway: you get used to the trains and sleep right through. A first-time guest jumps at the first whistle. Mice are the same: get one used to the signal slowly, then give a big dose, and it keeps eating. One that isn't used to it stops eating at once.

So how sick she gets depends on two things: how much the baby sends, and how used to it she is. One to three pregnant women in a hundred can't even keep water down. That's an illness, and it needs a hospital. Now back to the start: why does the dog keep eating?

One scientist compared the placentas of twenty-seven kinds of mammals. In our branch, humans, apes and macaques, the placenta makes this signal a hundred to a thousand times more than a mouse's does. Most other mammals barely make it. Some pregnant dogs eat less for a few days, but no one has studied it properly.

So the answer, for now: most animal babies barely send this letter. Human and ape babies send a lot. Why send it? One guess: in the early months the baby is building its heart and limbs. Steering mom away from strange food protects it. But the evidence for that guess is still weak. Scientists are now working on two new ideas: get mothers used to the letter before pregnancy, or seal the mailbox for a while.

What smell did you, or someone close to you, hate most while pregnant? Tell us in the comments.