We used to think our walks upright cost us dearly in the delivery room. Textbooks claimed it. Popular science claimed it. The story was simple: we stood up, our brains got big, and nature trapped us in an impossible squeeze. Evolution’s great compromise, they called it. A unique burden for our species alone.
Nicole Grunstra thinks we’ve been wrong for a long time.
Her study, fresh off the presses in Biological Reviews, pulls the rug out from under the “Obstetrical Dilemma.” It suggests difficult births aren’t a human curse. They’re a mammal trait. Period.
Not just us
Researchers at the University of Vienna didn’t look at just humans. They looked everywhere. Cows. Sheep. Whales. Deer. Seals. Even elephants. The data shows that dangerous deliveries are surprisingly common across the board. Wild animals, shaped by millions of years of brutal natural selection, still die from childbirth.
The mortality rates for female deer or antelopes during birth? They rival the numbers we see in human populations without modern medical access. The causes match up too. A baby too big for the canal. A fit that’s simply too tight.
Here is the kicker: whales have no bony pelvis to speak of. They don’t walk upright. And their calves still get stuck. How do you explain a whale stuck giving birth if the whole problem was just hips getting narrower? You don’t.
The trap of trade-offs
Why hasn’t evolution fixed this? If dying at birth is bad for survival, natural selection should weed it out.
It can’t. Because of the balance.
Larger babies have a better chance of living after they come out. But they’re harder to get out. Small babies get out easily. But they might freeze, or starve, or die within days. It’s a tightrope. One way you lose the mom. The other way you lose the kid.
Evolution doesn’t care if it’s difficult. It cares if something survives.
Dogs and pigs face a different math. Multiple babies mean more chaos. Small litters mean big, heavy pups that block the way. Large litters mean crowded canals with fetuses jamming into each other. Every strategy has its price.
No neat endings
Humans are just one flavor of this problem. Our challenge is specific—big brain, narrow hips shaped by walking on two legs. But cows have their own geometry. Their calves must push heads and forelegs through stiff bones all at once.
We aren’t exceptions. We’re examples.
The old idea that human birth was an anomaly needs to go. Reproduction is a high-risk game for almost everyone who has milk. The pelvis might look different, the baby might look different, but the tension is the same. We aren’t unique in our pain. Just in the shape of it.
Is it possible that every mammal is just holding its breath during delivery? Probably.
Grunstra, N. D. S. (2026). Humans are not unique: difficult birth is common in placental humans. Biological Reviews.
