The Placenta Is Not a War Zone

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Oct. 21 2011 7:11 AM

The Placenta Is Not a War Zone

Even though a father’s and mother’s genes differ, that doesn’t mean pregnancy is a “battle of the sexes.”

Happy parents couple holding a sonogram of their unborn child
Mothers and fathers have different genes, but that doesn't mean the womb is a war zone

Photograph by Yuri Arcurs/Hemera.

For every rom-com baby-delivery scene featuring a cursing expectant mother or sparring partners—think Katherine Heigl in Knocked Up or Ross and Rachel on Friends—there’s a scientist who insists that some aspect of pregnancy or fetal development is indeed a battle of the sexes.

The idea is that Mom and baby are not genetically identical, thanks to the genes baby gets from Dad. So they may find themselves at cross-purposes, at least in theory, with Dad’s genes pushing the fetus to demand greater resources from Mom and Mom’s genes struggling to prevent too dramatic a depletion. The latest example of this thinking comes from scientists at Yale, who argue that the fetus sets off a violent diversion in one area of the womb so that it can invade its mother’s tissue and establish a large enough placenta in another. It’s like the fetus’s placenta “blows up a bomb in the grocery story so it can go rob the bank a few blocks away,” the lead researcher, Harvey Kliman, told me (an image he’s fond of repeating). This is part and parcel of a normal pregnancy, he says. In fact, when the tactic doesn’t work, the result may be pre-eclampsia, a dangerous condition characterized by high maternal blood pressure that can require bedrest or lead to premature delivery.

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But the real deception may come not from the fetus but from the sensational language used to describe it. The interpretive leap from molecular minutia to all-out warfare isn’t actually well-substantiated. It’s also possible to see exquisite coordination, or cooperation, in this and other aspects of pregnancy—from changes in mom’s hormone production, food intake, and immune responses to the preparation for birth. Researchers have long debated the extent to which mother and fetus are adversaries or harmonious partners. But what’s at stake is not just the science of pregnancy but also its cultural interpretation, which has more widespread ramifications. The stories we tell ourselves about molecular events ought to reflect the plurality of metaphor—or else we should let them go.

The Yale group didn’t start with grandiose narrative. Instead, the researchers wanted to explain why women who have low levels of a particular protein early in pregnancy seem to be at higher risk for pre-eclampsia later. To do so, they tried to pinpoint what exactly this molecule, called placental protein 13, does under normal circumstances. In a healthy pregnancy, cells that come from the embryo’s placenta—called trophoblast cells—move into the walls of the uterus and help to open up maternal arteries, thereby increasing the available blood and nutrient supply. By studying tissue from abortions, the Yale group found that placental protein 13 was essentially absent near these crucial arteries. Instead, it had built up around the veins, in areas where the lining of the uterus appeared damaged.

Their interpretation? Placental protein 13 is responsible for a tactical diversion: It wreaks havoc in one place—creating a “total napalm war zone,” Kliman told me—to divert the mother’s immune “soldiers” and facilitate an attack on her arteries, making possible a larger placenta. What’s more, those invading cells are controlled by genes from the father and so reflect a drive for greater resources: “Dad would like the biggest baby possible, so he’s going to be aggressive in attacking blood vessels, while Mom is trying to make a smaller baby so she doesn’t die in childbirth,” says Kliman.

But the Yale group’s story about diversion and attack is not proved by their data—it is neither tested nor easily testable. Indeed, it is possible to look at the same cellular events and spin a less adversarial tale—say, that mother and fetus are collaborating to suppress the mother’s immune response, so that the fetus can do what it needs to do to build a healthy placenta. David Haig of Harvard, who suggests this alternate spin, also points out that by breaking down uterine tissue, placental protein 13 might actually be helping to facilitate the lateral expansion of the placenta, rather than simply causing a violent distraction. (Haig himself is well-known for advancing a conflict-based view of the placenta, though his metaphorical bent is gentler: The fetus makes demands on mother, and mother responds by “setting limits,” much as she’d do with a child, he says.) At the same time, the crucial assumption that Dad’s genes control the placenta and drive its demands on mom is by no means a given—see here, for example.)

The budding relationship between mother and fetus is, to some degree, open to interpretation—or pop psychological projection. Early on, for instance, cells around the embryo produce a hormone called human chorionic gonadotropin, which causes Mom to continue making the progesterone and estrogen necessary to sustain gestation. Fetal signals, acting directly or indirectly, also cause her to eat more and modify her immune responses so she won’t reject the pregnancy. Are these examples of the baby-to-be making bossy demands that will leave mom depleted, exhausted, and immune compromised? Or do they represent a form of exquisite communication, a priming of mother’s body that will help to ensure a healthy nine months?

Or what about this: Shortly before birth, mom’s brain ramps up production of oxytocin, the hormone responsible for uterine contractions. But she doesn’t produce more receptors for oxytocin—which would allow her body to respond and go into labor—until she receives a key signal from the fetal adrenal glands. In essence, the fetus tells Mom when it’s ready to go: “It’s a mechanism to prevent premature labor, before the fetus is fully mature, says Barry Keverne of the University of Cambridge. So again: pushy baby or sweet communication?

Ultimately, a storyline in which synchronicity trumps conflict has a lot to recommend it. Mother, father, and fetus share profound common interests: the success of the pregnancy, the birth of a healthy baby, and the continued survival of Mom. When scientists rev up the warfare rhetoric, it’s easy to forget that both parents are genetically related to baby, and that pregnancy has successfully perpetuated the species for a very long time.

I like a good prenatal metaphor as much as the next mom. But when the imagery gets this sensational, it’s easy to feel trapped in Hollywood clichés. Kliman’s careful, molecular work reveals something new about a key placental protein—why not appreciate it for that? Sometimes unadorned description—in this case, of how a tiny corner of the world really works—can be enough.

Amanda Schaffer is a science and medical columnist for Slate.