A new study from two University of Colorado researchers found that birth weights among U.S. infants have declined in recent years as Cesarean sections and labor inductions have become more common.

“Our data indicate that there has been a dramatic shift in birth timing in this country,” said Ryan Masters, one of the authors. “It is resulting in birth weight decline, and it is almost entirely due to changes in obstetric practices.”

The journal Demography published the findings on Jan. 29. Masters and co-author Andrea Tilstra were hoping to find why average birth weights began plummeting in 1990. The Centers for Disease Control and Prevention reports that low birth weights can cause infection or illness soon after birth, or lead to longer-term delays to development of motor and social skills.

The researchers analyzed 23 million births between 1990 and 2013 and discovered an association between low birth weights and medical decisions to end a pregnancy early.

“We found that the decline in birth weight would not have happened if it were not for the rapid increase in these obstetric interventions,” said Tilstra. “In fact, birth weights would have gone up.”

A range of factors, including financial incentives, could influence a physician into inducing labor sooner than 40 weeks. The average length of a pregnancy is now approximately 39 weeks, which matters because a fetus can put on weight in the last weeks of pregnancy.

“I hope it prompts physicians to take a step back and realize there can be broader public health impacts from these individual decisions,” said Tilstra. “And I hope it reminds mothers that they have more autonomy in the birth process.”