In the earliest days after birth, millions of bacteria make their home in a baby’s body — in the skin, mouth and especially the gut. These immigrants come from the birth canal and the mother’s feces (during a vaginal birth), the mother’s skin and mouth as she holds and nuzzles the baby and perhaps even from the placenta, although that source is still debated.

The colonizing microbiome can have a far-reaching impact on the baby’s health. Studies have suggested, for instance, that the populace of a baby’s microbiome in the first two years of life may predict later risk of obesity. Children born by cesarean section are also more likely to become obese, or to develop autoimmune diseases such as type 1 diabetes and asthma.

Lately , scientists have identified another major contributor to the infant microbiome. Breast milk, it turns out, is teeming with bacteria that colonize the infant’s gut, and could help set the course for the baby’s growing immune system and metabolism.

Moreover, breast milk seems to be rich in beneficial bacteria only when it comes directly from the mother’s breast — not even when the same milk is pumped and delivered later by bottle. Pumped milk still carries most other riches of breast milk. Nevertheless, this news about the microbiome, emerging from studies done in the past few years, taps into a fraught issue for many women: medical recommendations and societal pressure to breast-feed, combined with policies, or physical problems, that often make it all but impossible to do so .