Birth Weights Fell From 1990 to 2005

Mothers are giving birth to lighter babies in the U.S., and no one is quite sure why.

The 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 7 pounds, 7.54 ounces, a reversal of a trend that had seen birth weights climb steadily since the 1950s, according to the study. Babies were also born 2.5 days earlier on average in 2005 than in 1990, the study said. The small decrease in weight—based on an analysis of nearly 37 million nonmultiple 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.

The results are “surprising and unexpected,” said Emily Oken, an author of the paper and an assistant professor of population medicine at Harvard Medical School. “We do need to keep a close eye on the babies that are the smallest babies to see if the proportion of those is increasing over time, because they may require extra resources and have extra health concerns,” she said.

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 born at less than 37 weeks, according to Joann Petrini, senior adviser to 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 couldn’t be explained by common factors such as how much weight mothers gained during pregnancy, whether the delivery was induced or by cesarean section, the amount of prenatal care, or maternal-health issues such as smoking and hypertension, researchers said. Only babies born at between 37 weeks and 41 weeks of gestation, which doctors consider full term, were studied.

Researchers 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 wasn’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, 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 the 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 have a greater risk of high blood pressure and diabetes in the long run, said Dr. Oken.

From the 1950s until the 1980s, birth weights climbed as a result of increases in mothers’ weight and in the number of pounds they gained during pregnancy, as well as reduced smoking and older maternal age, according to Dr. Kramer.

http://online.wsj.com/article/SB20001424052748704423204575017471267586344.html

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