Antibiotic exposure before age two linked to increased risk of overweight and obesity in children

Key Takeaways:

• Antibiotic use within the first two years of life is associated with higher body mass index (BMI) and a significantly increased risk of overweight and obesity by age twelve.
• Antibiotic exposure before pregnancy, during pregnancy, or around the time of birth shows no statistically significant association with later childhood weight outcomes.
• Findings underscore the importance of careful antibiotic prescribing in early childhood, particularly avoiding unnecessary prescriptions for self-limiting conditions.

Children exposed to antibiotics during their first two years of life may face a heightened risk of developing overweight or obesity later in childhood, according to new research presented at the 2025 Paediatric Academic Societies (PAS) Meeting in Honolulu, Hawai‘i.

The study, led by investigators from the University of Oulu in Finland, specifically examined whether the timing of antibiotic exposure — ranging from before conception to early childhood — influenced weight outcomes in children. Researchers analysed data from 33,095 vaginally born children, drawing upon national health registers, medical records, and electronic growth charts to track weight development over time.

Early-Life Antibiotic Use Tied to Higher BMI and Obesity Risk

The research revealed that antibiotic use within the first 24 months of life was associated with significantly higher body mass index-for-age z-scores (zBMI) at two years of age. Children who received antibiotics during this window exhibited an average zBMI increase of 0.067 (95% confidence interval [CI]: 0.041 to 0.094) compared to children who were not exposed.

Long-term follow-up of these children up to the age of twelve showed that those who had received antibiotics in early life faced a 20% increased risk of developing obesity (adjusted hazard ratio [HR] 1.20; 95% CI, 1.10 to 1.31). Additionally, early antibiotic exposure was linked to a 9% increased risk of becoming overweight by age twelve. These associations persisted even after the researchers adjusted for several potential confounding variables, underscoring the robustness of the findings.

“Antibiotic exposure in the first two years of life has a stronger association with childhood weight gain than exposure during pregnancy stages or other early ages,” stated Sofia Ainonen, MD, PhD, a medical doctor at the University of Oulu and the study’s presenting author.

No Association Found from Maternal or Perinatal Exposure

The investigators also explored whether antibiotic exposure before conception, during pregnancy, or in the perinatal period influenced the likelihood of overweight or obesity in children. Their analysis indicated no statistically significant associations in these cases.

Within the study population, 39% of mothers had used antibiotics during the year preceding pregnancy, 27% during pregnancy itself, and 21% of children had been exposed during the perinatal period. However, none of these exposures were linked to higher BMI or weight-related outcomes later in childhood. This finding suggests that the critical window of influence may be limited to the early postnatal period.

Antibiotic Prescribing in Young Children Remains Widespread

Notably, the study found that 68% of the participating children had been prescribed antibiotics within their first two years of life, reflecting how common such early exposure remains. While antibiotics are essential for treating certain bacterial infections, they are frequently prescribed for conditions such as upper respiratory tract infections, where their clinical benefit may be minimal.

“Providers need to be cautious about prescribing antibiotics for young toddlers, especially unnecessary antibiotics for upper respiratory tract infections,” Ainonen emphasised.

Clinical and Research Implications

The findings contribute to a growing body of evidence indicating that the early postnatal period may represent a sensitive window during which antibiotic exposure can shape long-term health outcomes, including body weight regulation.

The study authors highlighted that future research aiming to uncover the biological mechanisms linking early antibiotic use to later obesity risk should concentrate specifically on the first two years of life, given the lack of associations found for exposures occurring before or during pregnancy.

Childhood obesity remains a critical global health challenge. As of 2022, more than 159 million school-aged children worldwide were diagnosed with obesity. The present study underscores the importance of antibiotic stewardship in early childhood as a potentially modifiable factor that could help reduce the long-term risk of overweight and obesity.

By focusing on appropriate prescribing practices and minimising unnecessary antibiotic use, especially during the crucial early years, healthcare providers may help support healthier long-term outcomes for children.

CCH Insight:

The results of this study are consistent with the increasing body of evidence that the gut microbiota plays a major role in metabolic health and conditions like obesity. A baby’s gut is sterile at birth, and then becomes populated with micro-organisms from its food and environment. This study supports the theory that oral ingestion of antibiotics during the first two years of life disrupts the establishment of a healthy gut microbiota and increases the risk of metabolic dysfunction and obesity.