Exposing babies in the womb to antibiotics could permanently weaken their immune systems and make them vulnerable to lung disease in later life, new research suggests.
The findings, from a study of mice, call into question the routine practice of prescribing antibiotics to women about to give birth by Caesarean section.
Gut bacteria wiped out by the drugs, which are given to prevent infection, play a key role in building a solid immune system in still-developing infant lungs, say the US scientists.
The research, published in the journal Science Translational Medicine, showed that even temporary disruption of gut bacteria made infant mice more likely to contract pneumonia and die.
Further investigation highlighted molecular signals released by bacteria colonising the gut which turned out to be critical to the immune system in the lungs.
The chemical messages told the lungs when to build immune cells, how many there should be, and where they should be deployed.
Lead scientist Dr Hitesh Deshmukh, from Cincinnati Children's Hospital Medical Centre, questioned the "near-automatic" practice in the US of prescribing antibiotics to women having Caesarean deliveries.
He said: "It is time to begin pushing back on practices that were established decades ago, when our level of understanding was different.
"To prevent infection in one infant, we are exposing 200 infants to the unwanted effects of antibiotics. A more balanced, more nuanced approach is possible."
In the UK, doctors' guidelines from the National Institute for Health and Care Excellence (Nice) say women undergoing Caesarean section should be offered prophylactic antibiotics "before skin incision".
The 2012 guidance adds: "Inform them that ... no effect on the baby has been demonstrated."
Many newborn babies in neonatal intensive care units also receive antibiotics.
The treatments protect against Group B Streptococci bacteria, the leading cause of deadly infections in newborns.
However, the drugs are indiscriminate and act against a wide range of bacteria, both good and bad.
Excess antibiotic use early in life may help explain why some people with no obvious genetic risk factors develop asthma or other lung diseases later in life, said Dr Deshmukh.
He added that the next step will be a clinical study to assess the safety and benefits of limiting antibiotic use among expectant mothers and newborn babies.