The millions of bacteria, fungi, viruses (human microbiome) that live on and within us are extremely important for our health in all sorts of ways. The birth experience (as the baby travels down the birth canal) is one way that a mother's microbes get transmitted to the baby ("seeds" the baby's microbiome). But babies born by Cesarean delivery start out picking up different species of microbes - from dust in the operating room.
Thus there has been concern with the possibility that a baby born by C-section, as compared to a vaginal delivery, will have life long microbiome differences. Swedish researchers studied this issue in 471 children and determined that by 5 years of age that the microbiome differences at birth and first year of life have generally disappeared. The differences in microbial composition had decreased to less than 2% in the 2 groups. This is good news!
Over the 5 years everyone had a great increase in the number of microbial species that live in the gut. The gut microbiome became more "adult-like", but it wasn't yet like that of adults. A conclusion was that not only does it take years to develop adult microbial complexity, but there is also individual variation in how long this takes. A person's diet, especially the introduction of solid foods, and environment all have an effect on species diversity and composition.
By the way, another way mothers transmit hundreds of species of microbes to their babies is during nursing in the breast milk (this is great!), and these species change over time. This is a good reason to breast feed - it's not just the nutrition, but also the microbes.
From Science Daily: Gut microbiota in Cesarean-born babies catches up
Infants born by cesarean section have a relatively meager array of bacteria in the gut. But by the age of three to five years they are broadly in line with their peers. This is shown by a study that also shows that it takes a remarkably long time for the mature intestinal microbiota to get established. ...continue reading "Five Year Olds Have Similar Gut Microbes, No Matter the Type of Birth"