1. What is a 9-month-old baby spitting up milk? 2. What are the reasons why 9-month-old babies spit up milk? 3. How to prevent 9-month-old babies from spitting up milk? 4. How to deal with a 9-month-old baby spitting up milk? 1. What is a 9-month-old baby spitting up milk? A 9-month-old baby begins to enter the complementary feeding period and will spit up milk at this time. Spitting up is when a baby regurgitates food from his mouth after feeding. This condition is common in newborns, but if it occurs at 9 months of age,