Thirteen-pound babies may make headlines, but they aren’t the norm. In fact, U.S. infants are getting smaller, according to researchers at the Harvard Pilgrim Health Care Institute’s Department of Population Medicine, an affiliate of Harvard Medical School. Their findings, published in the February issue of Obstetrics & Gynecology, suggest that birth weights in this country have declined during the past 15 years.

The study analyzed data on birth weight, maternal and neonatal characteristics, obstetric care, and other trends from the National Center for Health Statistics Natality Data Set, looking at 36,827,828 U.S. babies born at full-term between 1990 and 2005. Birth weight — a combination of fetal growth and length of gestation — was recorded in grams. The investigators teased out certain factors, including the mother’s age, race or ethnicity, education level, marital status, and tobacco use, as well as the amount of weight the women gained during pregnancy and how early in pregnancy they received prenatal care. They also considered the women’s risk of conditions such as hypertension and use of obstetric procedures such as induction of labor and Caesarean delivery.

Their findings came as a surprise. “Previous studies have shown that birth weights have increased steadily during the past half century,” said Emily Oken, Harvard Medical School assistant professor of population medicine. “We expected to see a continuation of those increases.” Higher birth weights have been attributed in part to women’s increasing age and weight and decreased smoking.

Instead, Oken and her colleagues found that birth weights had decreased by an average of 52 grams (1.83 ounces) between 1990 and 2005. Decreases were especially notable after 1995.

In contrast to previous research findings, birth weights fell even further in infants born to a subset of women considered to be at low risk for small babies. Mothers who were white, well-educated, married, didn’t smoke, received early prenatal care, and delivered vaginally with no complications had babies who weighed an average of 79 grams (2.78 ounces) less at birth during the study period.

The causes of this decline remain unclear. In addition to declines in birth weight, average gestation length among these full-term births also dropped by more than two days. “A logical conclusion might be that trends in obstetric management, such as greater use of Caesarean delivery and induction of labor, might account for these decreases in birth weight and gestation length,” said Oken. “However, our analysis showed that this was not the case.”

While the decline may simply represent a reversal of previous increases in birth weights, it may also be cause for concern. Babies born small not only face short-term complications such as increased likelihood of requiring intensive care after birth and even higher risk of death, but they may also be at higher risk for chronic diseases in adulthood.

Future research may identify factors not included in the current data that might contribute to lower birth weight, such as trends in mothers’ diets, physical activity, stress, and exposure to environmental toxins. “There’s still a lot we don’t know about the causes of low birth weight, said Oken. “More research needs to be done.”

The research was supported by grants from the National Institutes of Health.