Caesarean babies have an increased risk of becoming heavyweight adults.
An analysis of data on 38,000 individuals found those born by caesarean section were 22% more likely to be obese than those who had natural births.
They had a 26% greater chance of being overweight, as defined by their body mass index (BMI).
Scientists warned mothers to be aware of the possible long-term consequences of caesareans.
Effects of a surgery-assisted birth on a baby’s gut bacteria and genes could be two reasons for the trend, they believe.
Professor Neena Modi, from Imperial College London, said: “There are good reasons why C-section may be the best option for many mothers and their babies, and C-sections can on occasion be lifesaving. However, we need to understand the long-term outcomes in order to provide the best advice to women who are considering caesarean delivery.
“This study shows that babies born by C-section are more likely to be overweight or obese later in life. We now need to determine whether this is the result of the C-section, or if other reasons explain the association.”
Caesareans now account for up to a third of births in England, twice as many as in 1990. In some countries the C-section rate is much higher, with 60% of mothers in China and almost half in Brazil undergoing the procedure.
Previous research has linked other adverse outcomes in childhood including asthma and insulin-dependent Type 1 diabetes to caesarean birth. The new study pooled together results from 15 separate studies conducted in 10 countries.