Study finds decline in birthweight of full-term infants

Thirteen-pound
babies may make headlines, but they aren’t the norm. In fact, U.S. infants are
getting smaller, according to Harvard researchers at the Harvard Pilgrim Health Care
Institute’s Department of Population Medicine, an affiliate of Harvard Medical
School (HMS). 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, an HMS 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 beat 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.