Thirteen-poundbabies may make headlines, but they aren’t the norm. In fact, U.S. infants aregetting smaller, according to Harvard researchers at the Harvard Pilgrim Health CareInstitute’s Department of Population Medicine, an affiliate of Harvard MedicalSchool (HMS). Their findings, published in the February issue of Obstetrics &Gynecology, suggest that birth weights in this country have declined during thepast 15 years.The studyanalyzed data on birth weight, maternal and neonatal characteristics, obstetriccare, and other trends from the National Center for Health Statistics NatalityData Set, looking at 36,827,828 U.S. babies born at full-term between 1990 and2005. Birth weight — a combination of fetal growth and length of gestation — wasrecorded in grams. The investigators teased out certain factors, including themother’s age, race or ethnicity, education level, marital status, and tobaccouse, as well as the amount of weight the women gained during pregnancy and howearly in pregnancy they received prenatal care. They also considered thewomen’s risk of conditions such as hypertension and use of obstetric proceduressuch as induction of labor and Caesarean delivery.Their findingscame as a surprise. “Previous studies have shown that birth weights haveincreased steadily during the past half century,” said Emily Oken, an HMS assistant professor of population medicine. “We expected to seea continuation of those increases.” Higher birth weights have beenattributed in part to women’s increasing age and weight and decreased smoking.Instead, Oken andher colleagues found that birth weights had decreased by an average of 52 grams(1.83 ounces) between 1990 and 2005. Decreases were especially notable after1995.In contrast toprevious research findings, birth weights fell even further in infants born toa subset of women considered to be at low risk for small babies. Mothers whowere white, well-educated, married, didn’t smoke, received early prenatal care,and delivered vaginally with no complications hadbabies who weighed an average of 79 grams (2.78 ounces) less at birth duringthe study period.The causes ofthis decline remain unclear. In addition to declines in birth weight, averagegestation length among these full-term births also dropped by more than twodays. “A logical conclusion might be that trends in obstetric management, suchas greater use of Caesarean delivery and induction of labor, mightaccount for these decreases in birth weight and gestation length,” said Oken.“However, our analysis showed that this was not the case.”While the declinemay simply represent a reversal of previous increases in birth weights, it mayalso be cause for concern. Babies born small not only face short-termcomplications such as increased likelihood of requiring intensive care afterbirth and even higher risk of death, but they may also beat higher riskfor chronic diseases in adulthood.Future researchmay identify factors not included in the current data that might contribute tolower birth weight, such as trends in mothers’ diets, physical activity,stress, and exposure to environmental toxins. “There’s still a lot we don’tknow about the causes of low birth weight,”said Oken. “Moreresearch needs to be done.”The research wassupported by grants from the National Institutes of Health.