Birth weights in the United States are on the decline, a study has found. The report, released Thursday, found a small but significant decrease in average birth weights from 1990 to 2005, for reasons that scientists say are unclear.

The numbers, published in the February issue of the journal Obstetrics & Gynecology, mark a shift from earlier reports that noted a rise in birth weights in the latter part of the 20th century.

They also seem to go against conventional wisdom, experts said. In recent years, women have gotten larger, are smoking less and are older when they have children, all factors that contribute to higher birth weight in offspring.

The study looked at the records of 36,827,828 single full-term babies delivered between 1990 and 2005 in the United States, and it found an average decrease of 52 grams, or 1.83 ounces, in birth weight.

An even more dramatic decline was seen among a subset of women at low risk for small babies (women who were educated, married, white, didn't smoke, had early prenatal care and had vaginal deliveries with no complications). On average, this group's babies weighed 79 grams, or 2.79 ounces, less in 2005 than in 1990.

"We were startled by the findings," said senior author Dr. Emily Oken, assistant professor of population medicine at Harvard Medical School. "We tried really hard to explain it away, but we were unable to."

Details such as maternal age, race, education, tobacco use, when prenatal care started and weight gain during pregnancy did not explain the observed change, the authors said. Nor did changes in rates of cesarean or vaginal deliveries, whether labor was induced, whether ultrasounds were done or whether the mothers had medical conditions such as high blood pressure, eclampsia and diabetes.

The scientists did find that lengths of pregnancies got shorter over the study period, showing an average decrease of 2.4 days in the general population. That may have been because rates of cesarean sections and induced labor increased over time.

"If babies are born earlier, they're smaller because they have a little less time to grow," Oken said.

However, she added, the shortened pregnancy duration explained only part of the change. The same trend was seen in uninduced vaginal births delivered at full term.

Oken thinks the key to understanding the trend may lie in what wasn't studied.

"I think it could be some characteristics of the mothers that are changing that are not being measured, such as diet, physical activity, or some other types of behaviors," she said. Environmental factors may also figure in, as may the fact that rates of obese and overweight Americans are starting to plateau, which might affect the weight of their babies.

The small decline in birth weight isn't enough to be of concern yet, said Dr. Carol Major, professor of obstetrics and gynecology at UC Irvine and a board member of the Society for Maternal-Fetal Medicine, who was not involved in the study. "It's something that should be watched, but it's not likely at this point that, clinically, it's going to make a huge impact."

Underreporting of induced labor may be one key to the weight decline, said Joann Petrini, assistant director of research at Danbury Hospital in Danbury, Conn., and a senior advisor at the March of Dimes.

"Cesarean sections are very well reported, but induced labor is less well reported," said Petrini, who was not involved in the study.

The bigger issue, she said, is what the numbers could mean as the babies grow. Smaller babies are at a higher risk of death, and as adults they may be more prone to conditions such as heart disease.

"If we're seeing this decline in term babies, we need to stay vigilant on that, regardless of why it's happening," Petrini said.