During pregnancy the female body undergoes hormonal, metabolic, and immunological changes such as an increase in body fat early in pregnancy followed by reduced insulin sensitivity later in gestation. Pregnancy progression is also associated with dramatic alterations in the composition of the gut and vaginal microbiotas. The vaginal microbiota of pregnant women is characterized with a decrease in bacterial diversity which is also seen in the gut microbiota as pregnancy progresses. In the gut, the lower diversity is accompanied by an increase in “between sample” diversity and an increase in the number of Proteobacteria and opportunistic pathogens. Germfree mice inoculated with gut microbiota from pregnant women presented metabolic changes mirroring those of the pregnant women. This microbiota is then transferred to the infants in during birth and there are differences in the microbiota of the newborn depending on the mode of birth vaginal delivery vs. C-section. Whether the infant is breast-fed or formula fed also has a big impact on the infant’s microbiota.