Which brings us back to some of the other weird things about placentas. Like how the placenta is the only organ found in humans that’s temporary, in the sense that it is routinely created and discarded. And how it has its own unique microbiome, trillions of micro-organisms that are believed to influence how a newborn’s immune system works.

And how invasive it is—more so even than the common references to its parasitic qualities suggest. “The organ begins forming in the lining of the uterus as soon as a fertilized egg lands there, embedding itself deeply in the mother’s tissue and tapping into her arteries so aggressively that researchers liken it to cancer,” Denise Grady wrote in The New York Times last year. “In most other mammals, the placental attachment is much more superficial.”

Placental growth isn’t just aggressive in a way that reminds scientists of cancer; the way the organ develops actually mimics the spread of some cancers at the cellular level. Again, from The Times:

The front line of the invasion is a cell called a trophoblast, from the outer layer of the embryo. Early in pregnancy, these cells multiply explosively and stream out like a column of soldiers. ... [A]s trophoblasts invade, they alter certain proteins on their surfaces, called adhesion molecules, to become more motile. Researchers later found that cancer cells do the same thing as they spread from a tumor to invade other parts of the body.

Studying human placentas has long represented a major challenge to researchers—in part because it is risky for the fetus. Much of what we know about the placenta comes from animal models and lab-grown human cells. These approaches can be helpful, but they also entail limitations to understanding physiological processes in humans.

This element of the unknown may help explain the almost mystical quality imbued to the organ in several cultures. That, and the fact that serious health issues in pregnancy—for the mother and for the baby—can be traced back to the placenta. We may not understand the organ, but we know it is critically important. The instinct among some parents, then, is to save the placenta—or at least celebrate it somehow—rather than sending it off in a puddle of biohazardous waste to be incinerated.

Some women apparently freeze their placentas, or plant them beneath trees. Other women even (gulp) eat their placentas—although, as my colleague Julie Beck recently reported, there is no evidence that grilled placenta sandwiches or placenta smoothies or any other form of placentophagy is actually good for mothers. Eating a placenta, especially if uncooked, could be harmful. (Polenta, on the other hand, is delicious.)

The placenta may be, as the NIH claims, the “least understood human organ.” But there are research inroads being made. One of them is the Human Placenta Project, a $42 million NIH research effort. The project is bringing together a diverse group of placental biologists and other scientists to better understand what Romero calls a “marvelous and intriguing organ”—a single organ that does the work of many.