In the U.S., birth weights have dropped in the last few decades, and research points to the increase in cesarean deliveries and induced labor as the causes.
A study published in the journal Demography, shows that cesarean deliveries and inductions have shortened the average pregnancy. Fetuses can put on significant weight in the final weeks of pregnancy, which could explain why birth weights have been on the decline.
Researchers analyzed more than 23 million single births from 1990 to 2013. They gathered data on birth weight, week during which the delivery occurred and method of delivery.
They found the incidence of cesarean deliveries grew from 25 percent in 1990 to 31.2 percent in 2013, with delivery rates rising fastest in weeks 37 to 39. Labor induction more than doubled from 12 percent of deliveries to 29 percent during the study period.
Concurrently, the average length of pregnancy decreased by about one week, from 40 weeks to 39 weeks. Increasingly, births became concentrated between 37 and 39 weeks.
Between 1990 and 2013, the average birth weight of a U.S.-born babies fell by about 67 grams or about 0.14 pounds.
Researchers conducted a simulation and found that if the rates of cesarean delivery and induction hadn't increased, the average birth weight would have risen by 12 grams or 0.02 pounds.
Researchers said that while cesarean deliveries and inductions are sometimes medically necessary, providers should keep an eye on how often these interventions are used, since the consequences of low birth weights can be harmful. Low birth weight has been linked to negative effects on long-term health.