Stephanie Labarta, 31, was six months pregnant when she first noticed the occasional infant formula ad on Instagram and Facebook. Now, a year later, she sees ads three or four times a day, typically for Reckitt Benckiser's Enfamil or Nestle's Gerber Good Start.

“Gerber actually came up this morning on my feed - it started off with a contest to submit your smiling baby and then it trickled into the different types of formulas and what they have available," said Labarta, a senior analyst at a non-profit organisation in New York...