Babies born by Caesarean section are more at risk of tooth decay, research suggests.
The study looked at 156 babies
A New York University team found C-section babies were infected by a cavity-causing bacterium nearly a year earlier than those born naturally.
A study of 156 babies found those born vaginally were exposed to more bacteria during birth and built up resistance.
But the authors accepted a range of social factors also played a role, the Journal of Dental Research said.
The team found that on average the 29 C-section birth babies monitored as part of the study showed the first signs of the appearance of the bacterium, streptococcus mutans, after 17 months.
In comparison, the 127 natural birth babies only developed the bacterium at nearly 29 months of age, study said.
Streptococcus mutans grows on the surface of teeth and just above the gum line where it turns foods into acids. It is predominantly transmitted to babies from their mothers because of the close contact they have.
Previous studies have found that the earlier that streptococcus mutans develops, the higher the rate of cavities in children.
More than one in five births in the UK at the moment are via C-section.
Lead researcher Dr Yihong Li said: "Vaginally-delivered infants offer oral bacteria a less hospitable environment.
"They develop more resistance to these bacteria in their first year of life, in part because of exposure to a greater variety and intensity of bacteria from their mothers and the surrounding environment at birth.
"C-section babies have less bacterial exposure at birth, and therefore less resistance."
But he also pointed out that the C-section mothers in the study also had higher levels of tooth decay, a history of sexually transmitted infections and low family income, all of which could have contributed to the findings.
Roy Russell, professor of oral biology at the University of Newcastle, said it was important to find ways of minimising infection of the bacterium.
"It is the main decay causing bacterium and the earlier it gets a hold, the more damage it can cause."
He suggested the findings may persuade people that natural birth was best, but said other factors such as diet and oral care played a role that could cancel out high rates of infection.