Researchers are struggling to explain the latest findings on newborns' birth weight.
A Harvard study of nearly 37 million births shows U.S. newborns were slightly lighter in 2005 than in 1990, a surprising find as the obesity rate soars.
The study published in the journal "Obstetrics & Gynecology" ends a half-century of increasing birth rates. While the drop is less than two ounces, researchers are puzzled as to why it happened. Premature births, as well as twins and multiples, were excluded from the report.
The lead researcher in the study says babies are still bigger than they were a few decades ago, but the trend appears to have flat lined. An adviser to the March of Dimes says it's too soon to tell what the drop means, but that there should be vigilance about any decline in birth weights.
