Antibiotic treatment within 14 days of birth is associated with reduced weight and height in boys – but not girls – up to the age of six, new research suggests.

The study also found significantly higher body mass index (BMI) in both boys and girls following antibiotic use after the neonatal period, and within the first six years of life.

Researchers suggest the findings may be the result of changes in the development of the gut microbiome.

Professor Omry Koren at Bar-Ilan University, Israel, led the study together with Professor Samuli Rautava, of the University of Turku and University of Helsinki, in Finland.

Prof Koren said: “Antibiotics are vitally important and life-saving medications in newborn infants.

“Our results suggest that their use may also have unwanted long-term consequences which need to be considered.”

Researchers investigated the impact of neonatal antibiotic exposure in a cohort of 12,422 children born between 2008 and 2010 at the Turku University Hospital in Finland.

The babies had no genetic abnormalities or significant chronic disorders affecting growth and did not need long-term antibiotic treatment.

Antibiotics had been administered within the first 14 days of life to 1,151 (9.3%) of the babies in the study.

The researchers found that boys exposed to antibiotic treatment exhibited significantly lower weight as compared to non-exposed children throughout the first six years.

They also exhibited significantly lower height and BMI between the ages of two and six.

This observation was replicated in a German cohort, according to the study.

Researchers said that antibiotic exposure during the first days of life was associated with disturbances in the gut microbiome up until the age of two.

The researchers also demonstrated that germ-free male mice who were given the gut microbiome of antibiotic-exposed infants also displayed growth failure.

The scientists suggest these findings indicate a potential link between neonatal antibiotic exposure and impaired childhood growth, which may be a result of alterations caused by antibiotics in the composition of the gut microbiome.

The findings were published in the journal Nature Communications.

Dr Lindsay Hall, microbiome group leader at the Quadram Institute Bioscience, said: “Infants and children are often prescribed antibiotics to treat infections and this study … provides further evidence that this antibiotic exposure – particularly during the early life window, may impact developing microbial communities within the gut.

“What is interesting about this study is the team looked both when the children were young and then older and found that in the neonatal window both weight gain and height were reduced after antibiotic exposure – but only in boys (which they tested in animal models by giving them a faecal transplant).

“Older children – both girls and boys were then found to have a higher BMI, suggesting a complex relationship between antibiotic use and child development and metabolism.”

She added that the study opens up new avenues for investigating how antibiotics during infancy may alter the microbiome but warned that the results must be interpreted carefully as many daily factors may influence height and weight – like diet, which was not explored in detail in this study.

Dr Hall said: “They also only analysed a relatively small number in detail by looking specifically at their microbiome.

“More clinical studies and lab experiments are needed to understand how and why microbes like Bifidobacterium may be positively impacting early growth and development during early life.”