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

The numbers, published in the February issue of the journal Obstetrics & Gynecology, mark a shift from earlier reports, which had 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 new study looked at records of 36,827,828 single full-term babies who were delivered between 1990 and 2005 in the United States and found an average 1.83 ounce decrease in birth weight during that period.

A more dramatic decline was seen in a subset of women at low risk for small babies (women who were educated, married, white, didn’t smoke, had early prenatal care and vaginal deliveries with no complications). On average, this group’s babies weighed 2.79 ounces less in 2005 than 1999.

“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 and ethnicity, education, tobacco use, when prenatal care started, and weight gain during pregnancy did not explain the observed change, authors said. Nor did changes in rates of Caesarean or vaginal deliveries, whether labor was induced, if 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 Caesarean 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 only explained part of the change. The same trend was seen in uninduced vaginal births delivered at full term.