Is the placenta to blame?: evolutionary anthropologists take on postpartum hemorrhage

Given the importance of postpartum hemorrhage as a cause of death for women worldwide, we are interested in just about any discipline that tackles the issue from a new perspective. Though we've asked a lot of questions about postpartum hemorrhage and its etiology (the medical/scientific reason it happens; one question we had certainly never thought to ask was, "do other mammals and primates suffer from postpartum hemorrhage the way we humans do? It's kind of a startling question and two women, evolutionary anthropologists, recently took on. Elizabeth Abrams and Julienne Rutherford, recently had their work published in The American Anthropologist.

The answer is 'no' by the way. No, there is little evidence that other mammals or even primates suffer from postpartum hemorrhage the way we humans do.The authors hypothesize that the reason for this difference is that the human placenta operates in a more invasive manner than those of other primates.) For the laypeople among us, the placenta is essentially a large bag of vessels, a fetal organ that floats outside the fetus and interfaces with the mother's uterine wall to obtain nutrients for the fetus. In humans (and maybe the great apes – there just isn’t much data yet from chimpanzees or gorillas yet), the placenta actually invades the uterine wall-- cells from the placenta wrap around maternal blood vessels changing them or 'remodeling them.' In general the blood vessels of the mother are changed so that more blood, and therefore more nutrients, is shunted to the fetus. This process occurs early in the second trimester. Although always aggressive, Abrams and Rutherford hypothesize that in some cases the process really goes awry resulting in blood vessels that are too wide, too open and conducting too much blood to the placenta. Why is this important?

After the baby is born, the placenta next needs to be delivered. This is the critical window when postpartum hemorrhage often occurs. If the mother's uterus has been too invaded by the placenta, in other words if the vessels are widened, conducting too much blood, etc. it may be that this process then implicates itself in the separation of the placenta from the uterine wall. Either the uterus is not able to contract properly to expel the placenta or the reconfigured uterus somehow impedes placental separation. We know that most postpartum hemorrhage is due to inability of the uterus to contract properly (uterine atony).

So shall we sit back and watch evolution do its dirty work? The human species appears to have developed this invasive placenta that reconfigures the mother's anatomy for the benefit of the fetus. This enables our babies to have relatively large brains and bodies compared to the size of the mother and the birth canal. But Abrams and Rutherford suggest that the consequences could be catastrophic for mothers.

The best part about Abrams and Rutherford's hypothesis is their identification of the early second trimester as the time when the placental invasion occurs. They posit that at this point we could consider certain protein markers (from blood or urine) that could predict whether a mother is at risk of postpartum hemorrhage. This is quite an exciting concept-- instead of the current scenario where over 50% of postpartum hemorrhages have no risk factors we can begin to explore the possibility of a marker that would allow women to be identified early on. A fascinating new idea.


(image from here)

[http://education.yahoo.com/reference/gray/subjects/subject/12]