Late in pregnancy, some mothers-to-be are told they might be having a big baby, which makes many of them uneasy about what it means for their upcoming labor.
But a recent study published in the December issue of Maternal and Child Health Journal suggests medical professionals in America may significantly overestimate the number of women who are truly pregnant with large babies.
And that could contribute to an increased likelihood that they undergo interventions, such as C-sections or induction.
Researchers relied on the Listening To Mothers III Survey, a collection of 2,400 online interviews conducted with women who'd delivered a baby between 2011 and 2012. One question asked women if, near the end of their pregnancy, they'd been told by their medical provider that their baby might be getting "quite large."
Nearly one-third of the respondents answered "yes," yet only 9.9 percent of the women surveyed actually delivered a baby who weighed more than 8 pounds, 13 ounces -- a cutoff often used to indicate a baby is large.
"Medical information is sometimes limited by our technology," Dr. Ron Iverson, an assistant professor of obstetrics and gynecology at Boston University's School of Medicine and an author on the study, told The Huffington Post.
"Even though providers try to predict information such as fetal weight," he said, "we, as patients, need to understand that some data cannot be obtained exactly."
“Women who were told their babies may be large were more likely to ask for a planned C-section, and to have their labor medically induced.”
And yet simply the suspicion of large size seemed to be enough to influence whether pregnant women in the survey had certain interventions, the researchers found. Women who were told their babies may be large were more likely to ask for a planned C-section, and to have their labor medically induced. They were also more likely to ask for epidurals.
The authors do not make any claims that those interventions are bad; they could, in many cases, be necessary and welcome. But their findings do raise questions about what medical providers communicate to their patients -- and how their patients make sense of those conversations.
"It is possible that telling mothers their baby might be large could undermine their confidence that they can deliver their baby," said Erika Cheng, an assistant professor of pediatrics at Indiana University School of Medicine, and another author of the study.
"There is a lot of talk in the obstetric world about preventing the first C-section when it is medically possible," said Dr. Katherine Pasque, a professor of obstetrics and gynecology with the University of Michigan Health System who did not work on the new study.
"If someone can have a healthy vaginal delivery," she added, "that is far preferred." While a C-section can be life-saving and necessary, it is major surgery that carries some risk.
Of course, there are potential complications associated with delivering a large baby, including vaginal tearing, hemhorrhage and birth injuries to the newborn. Those risks increase when a baby's birth weight exceeds 9 pounds, 15 ounces.
“"There is a 15 to 20 percent risk of miscalculation of the fetal weight with ultrasound at the end of pregnancy."”
One 2008 study noted a 15 to 25 percent increase in large babies around the world. In the United States "fetal macrosomia" -- the medical term often used to describe a baby with a birth weight of greater than 8 pounds, 13 ounces -- occurs in roughly 8 percent of all births. Women with diabetes, for example, are at-risk for having larger babies, as are women with a personal history of having big newborns, Pasque added.
But the new study underscores that estimating a baby's weight during pregnancy is still an inexact science, and that medical care providers should remind mothers-to-be that estimates are imprecise.
"Unfortunately, there is a 15 to 20 percent risk of miscalculation of the fetal weight with ultrasound at the end of pregnancy," Iverson said, adding that physical exams produce similar results. "We have not been able to develop formulas or tools sensitive enough to give consistently correct weights."
"From a doctor's perspective, for me, the take-home message is to be careful about telling women they might have big babies!" Pasque said. "Because we have no idea."
CORRECTION: This article previously misstated that the increase in large infant births had been found in the "developed world"; the statistic in the original study referred to a broader population.
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