3.2 Early exposure to microorganisms

The gut microbiome starts to develop in a fetus during pregnancy and continues to change rapidly during the first 2-3 years of life (Roswall et al., 2021).

There are several factors which affect the microbiome of the developing fetus during pregnancy, including:

• maternal microbiota
• diet
• maternal genetics and epigenetics
• medications
• infections.

The mode of delivery can affect the microbiome of the newborn. Vaginal delivery can result in transference of the microorganisms from the mother’s vagina to the newborn, while caesarean section leads to exposure to microorganisms on the mother’s skin. A shorter or longer exposure time to the maternal microbiome during gestation can also influence the newborn’s gut microbiome, so gestational age can be important.

Finally, during childhood both maternal and environmental factors play an important role in the development of the gut microbiome. These include:

• breastfeeding versus formula feeding
• maternal diet if breastfed
• child’s genetics
• child’s environment (e.g., microorganisms on toys, family)
• family lifestyle (including stress)
• geographical location.

Figure 18 highlights some of the main factors during the three stages thought to influence a child’s microbiome.