Large birth weight forecasts often inaccurate, leading to more caesareans: study

 Photo: Getty Images

Katy Clemens wanted to have a natural, drug-free childbirth. Then an ultrasound suggested her baby would weigh 5.21kg (11.5lb), and her doctors insisted she schedule a caesarean section.

After describing the possible birth injuries that could befall such a large baby during a vaginal birth, Clemens relented. But when Sam was delivered by C-section, the doctors were in for a small surprise.

Sam weighed in at 3.54kg (7lb,13oz). The doctors were so surprised that they put him back on the scale to double check.

"They said the estimates are never off by more than a pound ... You imagined a baby that was 30 per cent bigger," said Clemens, who now says the surgery was unnecessary and probably caused her son to have breathing problems at birth, a complication more common in C-section babies.

Clemens is not alone. Last month, researchers reported that a significant number of women are wrongly told that they are having large babies. The research showed that mothers who believed they were having large babies were nearly five times more likely to ask for a scheduled C-section, even though the vast majority of their babies weighed less than 4kg (8lb, 13oz), which is the medical definition of a large baby.

Telling mothers their baby will be large "has a profound effect and contributes to undermining women's confidence they can deliver the baby," said Eugene Declercq, a professor at Boston University School of Public Health and one of the authors of the new paper, published in Maternal and Child Health Journal in December.

Fewer than 8 per cent of babies born in the United States meet the medical definition of a large baby. But exaggerated estimates of fetal birth weight may be playing a role in the nation's high caesarean section rates, Declercq said.

In the United States, nearly 1 in 3 babies are born by C-section.

"Childbirth is seen as a painful process to begin with," said Erika Cheng, an assistant professor of pediatrics who was the paper's lead author. "When you're told your baby will be large, it conjures up images of more pain, and risks and complications that might harm you and your baby. Women understandably might want to avoid that."


The new study, called 'Listening to Mothers III', was based on a nationally representative survey of some 1,960 new mothers. It found that 4 out of 5 of the mothers who were warned they might have large babies gave birth to infants who were not large and weighed less than 4kg.

Yet these mothers were almost twice as likely to have medical interventions such as having doctors medically induce their labour or attempting to self-induce labour, presumably out of concern the baby would continue growing otherwise. They were also nearly twice as likely as other mothers to have a planned caesarean, although the increase fell just short of being statistically meaningful.

Guidelines from the American College of Obstetricians and Gynecologists in 2014 say that a suspected large baby is only "rarely" an indication for a caesarean delivery. The guidelines note that newborns weighing 4.9kg (11lb) also are rare, although they leave open the possibility of performing a C-section delivery for a 4.9kg baby, or a 4.5kg (10lb) baby whose mother is diabetic.

The concerns about vaginal delivery of large babies are real. Large babies born vaginally are at risk of shoulder dystocia, which is when the head is delivered but the shoulders are stuck. This can result in nerve damage that causes a loss of movement or weakness in the arms. But the guidelines urge caution in the use of ultrasound scans to estimate foetal weight, noting that the scans are associated with caesareans, and they urge doctors to use them "sparingly" and to counsel patients that scans are imprecise during the third trimester.

"We really urge caution because our tools for identification of a large fetus are so poor," said Dr Aaron Caughey, who helped develop the 2014 guidelines. "We just aren't very good at predicting birth weight - every study in the literature would tell you that."

Although the guidelines say doctors should "offer" a caesarean to women with a baby predicted to weigh 11 pounds, he said, "I have a sense that many clinicians do it at lower thresholds than that," which may be keeping C-section rates up.

"We don't want people pulling the trigger at every estimate above 4kg," he said.

While it is important to share information with patients, the mere act of "offering" C-sections influences women's decisions, Caughey said. "If you're offering something that isn't normally provided, you are saying somehow that this is a pretty good option."

New York Times