New research says that bottle feeding appears to increase the risk of babies developing hypertrophic pyloric stenosis, a form of stomach obstruction characterized by severe and frequent projectile vomiting and most common among infants in their first 2 months of life.

Surgery, called pyloromyotomy, may be needed to clear the hypertrophic pyloric stenosis (HPS), which occurs when the smooth muscle layer of the pylorus (the “gateway” between the stomach and small intestines) thickens.

Dr. Jarod P. McAteer from Seattle Children’s Hospital and colleagues say in the study that although this is a fairly common condition – it occurs in approximately 2 in 1,000 births – the cause remains unknown.

This study, published in JAMA Pediatrics, sets out to establish if bottle feeding during early infancy increases the risk of developing HPS, and if so, how the risk is modified by other factors.

The study reports that breastfeeding became a target of interest in the 1980s, as researchers noted that an upsurge in the breastfeeding rates coincided with a decrease in the number of cases of HPS.

The doctors claim that this study analyzes the largest group of HPS patients with data on infant feeding practices yet studied.

The researchers collected data from Washington State birth certificates and hospital discharge data between January 1, 2003, and December 31, 2009.

Participants in the study had to be singleton babies under 6 months old who had been both diagnosed with HPS and undergone corrective surgery.

During the time period selected, researchers identified 714 such infants and matched them to “control” babies – ones without HPS.

The researchers do note that as the study is observational, all associations should be interpreted “with caution.” They also point out the data of how a baby was fed is based on information provided when the mother and baby were discharged from the hospital.

Some babies who were breastfed initially could have switched to bottle feeding (or vice versa) prior to developing HPS, the researchers say.

There are many reasons why mothers stop breastfeeding their babies, including cracked and sore nipples or mastitis. A report, published by UNICEF, reports that 60% of women did not manage to breastfeed for as long as they would have wanted.

Another potential pitfall in this research is that babies bottle-fed expressed milk are coded as bottle-fed in the data, so interpretations regarding formula milk versus breast milk may also be skewed.

The researchers found that the incidence of HPS decreased from 14 per 10,000 births in 2003 to 9 per 10,000 births in 2009. They note that the popularity of breastfeeding also increased during that time from 80% in 2003 to 94% in 2009.

The study revealed that bottle-fed infants were more likely to develop HPS, compared with controls (19.5% vs. 9.1%). The odds of an infant developing HPS also increased if they were male, and when mothers were 35 years and older and multiparous (having given birth more than once before.).

The study concludes:

These data suggest that bottle feeding may play a role in HPS etiology, and further investigations may help to elucidate the mechanisms underlying the observed effect modification by age and parity.”

In an accompanying editorial, Dr. Douglas C. Barnhart from the Primary Children’s Hospital, Salt Lake City, writes:

“While the data seem convincing that bottle feeding increases the risk, the reason is not clear.”

“Further understanding of the pathogenesis of hypertrophic pyloric stenosis will come from both basic research and more detailed epidemiologic studies,” Barnhart concludes.

Dr. McAteer and his colleagues agree, adding:

“Further studies are warranted to validate these findings and to look more closely at the speculative mechanisms, including possible hormonal effects, underlying the bottle feeding-HPS association.”