Mothers are giving birth to lighter babies in the U.S., and no one is quite sure why.
This finding, published Thursday in the Journal of Obstetrics and Gynecology, has potentially troubling public-health implications, if the trend continues. Low-birth-weight babies are at higher risk for a host of health problems.
Between 1990 and 2005, the birth weight of full-term babies in the U.S. declined nearly two ounces to an average of seven pounds and 7.54 ounces, a reversal of a trend that had seen birth weights climb steadily since the 1950s, according to the study. They were also born 2.5 days earlier on average in 2005 than in 1990, the study said.
The decrease in weight—based on an analysis of nearly 37 million non-multiple births from a national database—isn't likely to affect the health of the average baby in the study, according to researchers. But the data showed a 1% increase in the number of the lowest-weight babies and suggested the birth-weight decline didn't stop in 2005.
These data suggest that it may be important for medical professionals to pay attention to the weight of babies born around 37 weeks and 38 weeks, as well as those considered pre-term, or less than 37 weeks, according to Joann Petrini, senior adviser at the March of Dimes and assistant research director at Danbury Hospital in Connecticut, who wasn't involved in the study.
Researchers also found a 2% decrease in the number of babies considered large—those over the 90th percentile of weight for gestational age—which is a positive, according to Dr. Oken. Large babies can experience more birth trauma and cause more birth injury to the mother.
The lower-birth-weight trend could not be explained by common factors like how much weight mothers gained during pregnancy, whether the delivery was induced or by cesarean section, prenatal care, or common maternal-health issues such as smoking and hypertension, researchers said.
Researchers also repeated their analysis in a sample of low-risk women—healthy, educated Caucasians in their mid-to-late 20s—and found that the decrease in birth weight was even more pronounced, suggesting that the trend isn't the result of changes in the population of mothers.
Other investigators also have begun to note the same trend. "There's no question" about the change in birth-weight pattern, said Michael Kramer, scientific director of the Institute for Human Development and Child and Youth Health at the Canadian Institutes of Health Research, who wasn't involved in the study.
"It is a new trend," he said. "We really don't know why the birth weight has decreased." A similar pattern has been observed in Canada, he said.
Some potential factors that weren't examined in this study include better control of gestational diabetes—when a mother develops diabetes during pregnancy—and more physical activity during pregnancy, said Dr. Kramer.
Babies considered too large, as well as too small, tend to have more health problems in the long run. The optimal size for a newborn is around 4,000 grams, or roughly 8.8 pounds, according to Dr. Kramer. The average baby in the study was found to be smaller than optimal. In 1990, the average birth weight was 3,441 grams, and in 2005 it fell to 3,389 grams, according to the study.
Babies born too small tend to have higher blood pressure and a greater risk of diabetes in the long run, said Dr. Oken.
From the 1950s until the 1980s, birth weights had increased as a result of increases in mothers' weight and how many pounds they gained during the pregnancy, as well as reduced smoking and older maternal age, according to Dr. Kramer.Source