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Researchers' best guess is that a flu infection causes stress in the mother, which might in turn affect fetal development. During pregnancy, a woman's heart and lungs are working substantially harder than usual, and her immune system is compromised, so a few infections (like influenza) may potentially become more intense. Although most pregnant women who get the flu survive with no serious problems, they are still more likely than other healthy adults to also develop respiratory failure and secondary bacterial infections like pneumonia — potentially fatal conditions that may require hospitalization and mechanical ventilation. "It is these severe cases that are dangerous for both the mother and her baby," said Harrison in an e-mail message.
The idea that environmental conditions in the womb may have lifelong effects on the fetus is certainly not new. British epidemiologist D.J. Barker first proposed his theory of fetal origins in 1992, arguing that when the fetus doesn't get enough nutrition in utero, for example, an increased risk of future heart disease and diabetes somehow gets "programmed" into his or her development. There wasn't very much data to back Barker's theory at the time, but over the decades, a wealth of animal and human data has suggested it's true. Maternal conditions like high blood pressure and diabetes and behaviors like smoking and drinking have all been identified as factors that can harm the fetus. Each risk factor may lead to various long-term consequences, including mental retardation, low birth weight or an increased risk of heart disease, diabetes or schizophrenia.
But the flu-stress theory is still just a theory. There is only epidemiological evidence to support it; a clinical trial measuring the effects of flu-induced maternal stress would, of course, be unethical. And the link could involve any number of unknown variables: in the new study co-authored by Finch, it's not even clear which of the survey respondents' mothers actually caught the flu, because that information was not available.
Still, the study's authors argue that there are few other immediately obvious alternative hypotheses, given the cluster of outcomes among babies born at certain times. "Why is it that only those born in 1919 showed the spike [in heart disease]?" asks study author Douglas Almond, a professor of economics at Columbia University and a pioneer in applying the fetal-origins theory to economics. "People who were born just before and after the flu should be affected as well."
Whatever the exact biological pathway, for doctors like Harrison, the findings have practical relevance: they reinforce the importance of getting a flu shot, especially for pregnant women, many of whom say they are reluctant to receive the new 2009 H1N1 vaccine. "I am already 37 weeks into my pregnancy and haven't fallen sick and have been healthy all along, and I don't see the point of introducing a foreign body into my body," says first-time mother Laurie Koch-Smith, 41, in Westchester County, New York, who says she thinks the risk of H1N1 infection has been overhyped.
But even if pregnant moms avoid catching the H1N1 flu, the vaccine has other benefits, says Harrison. "The baby of a woman who got the influenza vaccine [during pregnancy] will be born with antibodies to influenza," she says, adding that immunity — albeit temporary — would greatly reduce the chances of the infant coming down with the flu during the first few months of life.