BOULDER, Colo. — For decades, birthweights for babies born in the U.S. were rising. But something shifted around 1990, when birthweights of newborns began to plummet. The trend now concerns public health officials who see a less-robust future society if the declining birthweight pattern continues.
So what is causing otherwise healthy women to deliver babies that are smaller than previous generations? Scientists with the University of Colorado Boulder set out on a mission to discover the answer.
“Our data indicate that there has been a dramatic shift in birth timing in this country,” says senior study author Ryan Masters, a social demographer with the Institute of Behavioral Science, in a statement. “It is resulting in birthweight decline, and it is almost entirely due to changes in obstetric practices.”
Researchers studied more than 23 million single birth records reported to the National Vital Statistics System from 1990 to 2013. They plotted various elements including birthweight, number of weeks into pregnancy at delivery and the method of delivery — vaginal, induced vaginal, cesarean or induction/cesarean.
They sought to understand what would have occurred if the cesarean and induction rates had not increased over that 23-year timespan.
“We found that the decline in birthweight would not have happened if it were not for the rapid increase in these obstetric interventions,” says lead author Andrea Tilstra, a PhD candidate in the university’s Department of Sociology. “In fact, birthweights would have gone up.”
Researchers found that from 1990 to 2013, the average length of pregnancy dropped from 40 to 39 weeks, with most births concentrated between 37 and 39 weeks. Far fewer births stretched into 42 weeks, once considered a common benchmark for the longer end of a healthy delivery range.
Meanwhile, cesarean deliveries became much more common, increasing from 25% in 1990 to 31.2% in 2013. Rates for C-sections rose fastest for healthy mothers-to-be delivering in weeks 37 to 39. Labor induction more than doubled over the same timespan from 12% to 29% of deliveries.
Researchers determined that about 18% of vaginal births in 2013 would have occurred later in the pregnancy had the deliveries happened in 1990, when inductions were much less common. Study authors say this bonus time in utero makes a difference, because a fetus often gains weight at the most rapid rate in the final weeks of pregnancy.
Researchers found that the average baby born in the United States in 2013 arrived weighing 67 grams (2.363 ounces) less than the average newborn in 1990. Had rates of obstetric interventions remained constant, researchers say birthweights would have risen 12 grams (0.423 ounce) over the 23-year study period.
“By intervening in the pregnancy instead of allowing it to reach its natural finality we are shifting when birth happens, and that can have public health consequences,” says Tilstra.
Concerns about birthweight stem from the association between a lower birthweight and poorer long-term health outcomes as well as lower educational achievement.
The authors compared the relaxed guidelines of the American College of Obstetricians and Gynecologists with those of the World Health Organization (WHO) in matters of obstetric interventions. While U.S. doctors are granted much leeway in making these decisions, WHO actively discourages physicians in other countries from undertaking obstetric interventions.
The study authors stress that while they do understand the medical necessity of many inductions and cesarean deliveries, they wonder what is driving the increasing trend in the U.S. to interfere with natural deliveries. Could it be cultural, institutional or even financial incentives at the hospital or insurance level? And why have the increases in obstetric interventions occurred mostly among healthy women who are at full-term but not yet overdue?
Researchers emphasize that it is not their intention to interfere in medical decisions between doctors and patients. They simply want the information from this study to be part of the decision-making discussion.
“I hope it prompts physicians to take a step back and realize there can be broader public health impacts from these individual decisions, and I hope it reminds mothers that they have more autonomy in the birth process than they sometimes feel they do,” Tilstra concludes. “If something is not obviously medically necessary it is important to ask why it’s happening.”
The study is published in the journal Demography.