Phase one

Intrauterine period

The fetus becomes exposed to maternal microbiota through transplacenta passage into amniotic fluid

Phase two

1^st week of life

The newborn ingests maternal vaginal/colonic microbiota with passage through the birth canal (full term, vaginal delivery)

Phase three

Two weeks to four months

Introduction of oral liquid feedings

Phase four

Four months to one year

Period of weaning to solid foods

Phase five

One to three years

Infant receives table food and intestinal microbiome resembles that of adult intestine (diversity of bacteria with greater than 1000 species)