While it’s true that it provides the developing fetus with oxygen and nutrients, no other animal has a placenta that exacts the same biological toll. Women have the thickest uterine lining of all mammals, so to get deep enough to have access to the blood supply, the human placenta must be very aggressive — think of this as an evolutionary arms race. (And one that can lead to some scary complications, like pre-eclampsia.)

Placental extract seems a less common ingredient in today’s moisturizers. A few of my patients began to ask if they might take their placenta home to bury, a custom in some cultures.

And then patients started to ask about eating placentas.

As someone who specializes in infectious diseases and sexual health, I am used to hearing things about the body that most people can’t imagine. Yet I admit I was dumbfounded.

Placentas are often colonized with bacteria. Many are infected. As a general rule it’s best not to eat something that is potentially teeming with bacteria, many of which may be pathogenic (meaning they can cause disease).

Why might a woman eat her placenta? I asked.

Mammals do it, I was told.

Sigh.

It’s true that many mammals eat their placenta. But there are a lot of differences between us and other mammals: Other mammals often have litters. Or differently shaped uter i with less invasive placentas. They also mostly have estrus — not menstrual — cycles, meaning they typically only have sex when in heat.