Annie Murphy Paul: What we learn before we're born (2)
Now it turns out that fetuses are learning even bigger lessons. But before I get to that, I want to address something that you may be wondering about. The notion of fetal learning may conjure up for you attempts to enrich the fetus -- like playing Mozart through headphones placed on a pregnant belly. But actually, the nine-month-long process of molding and shaping that goes on in the womb is a lot more visceral and consequential than that. Much of what a pregnant woman encounters in her daily life -- the air she breathes, the food and drink she consumes, the chemicals she's exposed to, even the emotions she feels -- are shared in some fashion with her fetus. They make up a mix of influences as individual and idiosyncratic as the woman herself. The fetus incorporates these offerings into its own body, makes them part of its flesh and blood. And often it does something more. It treats these maternal contributions as information, as what I like to call biological postcards from the world outside.
So what a fetus is learning about in utero is not Mozart's "Magic Flute" but answers to questions much more critical to its survival. Will it be born into a world of abundance or scarcity? Will it be safe and protected, or will it face constant dangers and threats? Will it live a long, fruitful life or a short, harried one? The pregnant woman's diet and stress level in particular provide important clues to prevailing conditions like a finger lifted to the wind. The resulting tuning and tweaking of a fetus' brain and other organs are part of what give us humans our enormous flexibility, our ability to thrive in a huge variety of environments, from the country to the city, from the tundra to the desert. To conclude, I want to tell you two stories about how mothers teach their children about the world even before they're born. In the autumn of 1944, the darkest days of World War II, German troops blockaded Western Holland, turning away all shipments of food. The opening of the Nazi's siege was followed by one of the harshest winters in decades -- so cold the water in the canals froze solid. Soon food became scarce, with many Dutch surviving on just 500 calories a day -- a quarter of what they consumed before the war. As weeks of deprivation stretched into months, some resorted to eating tulip bulbs. By the beginning of May, the nation's carefully rationed food reserve was completely exhausted. The specter of mass starvation loomed. And then on May 5th, 1945, the siege came to a sudden end when Holland was liberated by the Allies.
The "Hunger Winter," as it came to be known, killed some 10,000 people and weakened thousands more. But there was another population that was affected -- the 40,000 fetuses in utero during the siege. Some of the effects of malnutrition during pregnancy were immediately apparent in higher rates of stillbirths, birth defects, low birth weights and infant mortality. But others wouldn't be discovered for many years. Decades after the "Hunger Winter," researchers documented that people whose mothers were pregnant during the siege have more obesity, more diabetes and more heart disease in later life than individuals who were gestated under normal conditions. These individuals' prenatal experience of starvation seems to have changed their bodies in myriad ways. They have higher blood pressure, poorer cholesterol profiles and reduced glucose tolerance -- a precursor of diabetes.
Why would undernutrition in the womb result in disease later? One explanation is that fetuses are making the best of a bad situation. When food is scarce, they divert nutrients towards the really critical organ, the brain, and away from other organs like the heart and liver. This keeps the fetus alive in the short-term, but the bill comes due later on in life when those other organs, deprived early on, become more susceptible to disease.
But that may not be all that's going on. It seems that fetuses are taking cues from the intrauterine environment and tailoring their physiology accordingly. They're preparing themselves for the kind of world they will encounter on the other side of the womb. The fetus adjusts its metabolism and other physiological processes in anticipation of the environment that awaits it. And the basis of the fetus' prediction is what its mother eats. The meals a pregnant woman consumes constitute a kind of story, a fairy tale of abundance or a grim chronicle of deprivation. This story imparts information that the fetus uses to organize its body and its systems -- an adaptation to prevailing circumstances that facilitates its future survival. Faced with severely limited resources, a smaller-sized child with reduced energy requirements will, in fact, have a better chance of living to adulthood.
The real trouble comes when pregnant women are, in a sense, unreliable narrators, when fetuses are led to expect a world of scarcity and are born instead into a world of plenty. This is what happened to the children of the Dutch "Hunger Winter." And their higher rates of obesity, diabetes and heart disease are the result. Bodies that were built to hang onto every calorie found themselves swimming in the superfluous calories of the post-war Western diet. The world they had learned about while in utero was not the same as the world into which they were born.
Here's another story. At 8:46 a.m. on September 11th, 2001, there were tens of thousands of people in the vicinity of the World Trade Center in New York -- commuters spilling off trains, waitresses setting tables for the morning rush, brokers already working the phones on Wall Street. 1,700 of these people were pregnant women. When the planes struck and the towers collapsed, many of these women experienced the same horrors inflicted on other survivors of the disaster -- the overwhelming chaos and confusion, the rolling clouds of potentially toxic dust and debris, the heart-pounding fear for their lives.
About a year after 9/11, researchers examined a group of women who were pregnant when they were exposed to the World Trade Center attack. In the babies of those women who developed post-traumatic stress syndrome, or PTSD, following their ordeal, researchers discovered a biological marker of susceptibility to PTSD -- an effect that was most pronounced in infants whose mothers experienced the catastrophe in their third trimester. In other words, the mothers with post-traumatic stress syndrome had passed on a vulnerability to the condition to their children while they were still in utero.
Now consider this: post-traumatic stress syndrome appears to be a reaction to stress gone very wrong, causing its victims tremendous unnecessary suffering. But there's another way of thinking about PTSD. What looks like pathology to us may actually be a useful adaptation in some circumstances. In a particularly dangerous environment, the characteristic manifestations of PTSD -- a hyper-awareness of one's surroundings, a quick-trigger response to danger -- could save someone's life. The notion that the prenatal transmission of PTSD risk is adaptive is still speculative, but I find it rather poignant. It would mean that, even before birth, mothers are warning their children that it's a wild world out there, telling them, "Be careful." Let me be clear. Fetal origins research is not about blaming women for what happens during pregnancy. It's about discovering how best to promote the health and well-being of the next generation. That important effort must include a focus on what fetuses learn during the nine months they spend in the womb. Learning is one of life's most essential activities, and it begins much earlier than we ever imagined.