Katy Clemens wanted to have a natural, drug-free childbirth. Then an ultrasound suggested her baby would weigh 11.5 pounds, and her doctors insisted she schedule a cesarean 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 7 pounds 13 ounces — nearly four pounds less than expected. 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, now 37, a lawyer from Columbia, 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 4,000 grams (8 pounds 13 ounces), 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 R Declercq, a professor at Boston University School of Public Health who is one of the authors of the new paper, published in Maternal and Child Health Journal in December.

“Childbirth is seen as a painful process to begin with,” said Erika R Cheng, an assistant professor of pediatrics at Indiana University School of Medicine in Indianapolis 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 Childbirth Connection’s nationally representative survey of some 1,960 new mothers. It found that four out of five of the mothers who were warned they might have large babies gave bi-rth to infants who were not large (less than 8 pounds 13 ounces).

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 cesarean, though the increase fell just short of being statistically meaningful.

Ultrasound scans

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 from the American College of Obstetricians and Gynecologists in 2014 urge caution in the use of ultrasound scans to estimate foetal weight, noting that the scans are associated with cesareans, 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 foetus are so poor,” said Dr Aaron B 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.”

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

Some mothers, like Clemens, say the “offer” comes with a lot of pressure. “I was bullied into the C-section,” she said. “They were telling me my baby would have permanent nerve damage and would never be able to dance or throw a ball because I had a fantasy of what a birth is like, and ‘Isn’t the only thing that matters a healthy baby?’ I felt like I had very little choice.”

Not all mothers are given a hard sell, however. Michelle Mirsky, 30, a psychologist from Hartford who was told her baby could weigh 9.5 pounds, said doctors discussed the possibility of a C-section but did not pressure her. “But I definitely didn’t want a C-section; I had a lot of family and friends who had C-sections, and I knew the recovery was not a piece of cake.”

When her daughter Aliza was born, she weighed only 7 pounds 8 ounces. “She was perfectly average,” Mirsky said. “I barely got to the hospital on time.”