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Biology

The Evolution of Placental Diversity and Maternal–Fetal Conflict in Mammals

Quick fact

Placentas vary more than any other mammalian organ: from a diffuse, horseshoe-like shape in pigs to a single round disc in humans. Even more surprising, the number of tissue layers between maternal and fetal blood can range from six down to just one, allowing different species to control how aggressively the fetus draws resources from its mother.

Why this is interesting

Every mammal—including you—was once nourished by a temporary organ that is more varied in shape and function than almost any other body part. Why do some placentas look like a balloon, while others resemble a flat cake?