According to a study by the American Society for Microbiology, after the age of nine months, the development of the infant gut microbiota is driven by the transition to family foods, not maternal obesity.
“Our results reveal that the transition from early infant feeding to family foods is a major determinant for gut microbiota development,” said the senior author. “Maternal obesity did not influence microbial diversity or specific taxon abundances during the complementary feeding period.”
Children are basically born without microbes in their gut and immediately after birth are colonized. Each individual has a unique microbiome and it is more of less like a fingerprint. Later in life, it can change in response to diet, but only slightly.
For more information, read the full study.
https://www.sciencedaily.com/releases/2016/02/160211142225.htm