We've known for a while that exposure to microbes during the baby's first year is important (and good) for the developing immune system. In addition, recent research suggests that during the second trimester of pregnancy the developing fetus already has microbes in its body which help "educate" the developing immune system.
An international team of researchers found that bacterial species are present in the fetus during the second trimester. They examined and found bacteria in the gut, lungs, skin, and placenta of the fetuses. Gardnerella, Lactobacillus, and Staphylococcus species were found in most of the fetal organs. Streptococcus, Enterococcus, and Prevotella species were also frequently found, as well as other species. The view is that the microbes came from the mother ("vertical transmission of microbes").
What the researchers found exciting was not just that there were microbes living already in the fetus in the second trimester, but that these microbes "educate" the developing immune system, especially memory T cells. This prepares the newborn with all sorts of exposures (including harmful microbes) once it is born.
The researchers used genetic sequencing (16S ribosomal RNA sequencing) on fetal and placental tissues, which were obtained from second trimester (starting with week 13) abortions.
Excerpts from The Scientist: Microbes in Human Fetuses Spur immune Development
Over the last decade, scientists have shown that the fetal immune system comes online much sooner than was initially thought, but what type of antigens train nascent immune cells and how this affects subsequent development remain open questions. In a study published June 1 in Cell, researchers determined that second-trimester human fetuses harbor live bacteria in tissues all over their bodies that can activate fetal T cells. ...continue reading "Bacteria Are In the Developing Baby By the Second Trimester"