Babies born in the United States weigh less than they used to, with the average birth weight dropping 2.4 ounces (0.15 pounds) — from 7.31 pounds to 7.16 pounds — from 1990 to 2013, according to a new analysis using data from more than 23 million births. In that time, births were also less likely to occur after the 40th week of pregnancy, often considered the official due date (18 percent in 2013 vs. 29 percent in 1990) and more likely to occur in weeks 37 to 39 (49 percent in 2013 vs. 38 percent in 1990). Researchers from the University of Colorado at Boulder who analyzed data from the Centers for Disease Control and Prevention attributed the drop in birth weight primarily to shorter pregnancies and earlier births because of increased scheduling of Caesarean sections and induced labor before week 40. The percentage of births by C-section grew from 25 percent in 1990 to 31 percent in 2013, and deliveries that involved induced labor went from 12 percent to 29 percent. The report does not speculate on the reasons for this. The lower average birthrate still leaves the average baby well above the 5 pounds 8 ounces that is considered a low birth weight and that increases a child’s risk for health problems.