1. What is a 4-month-old baby spitting up milk? 2. What are the reasons why a 4-month-old baby spits up milk? 3. How to prevent and deal with vomiting in a 4-month-old baby? 4. What are the health effects of spitting up milk for a 4-month-old baby? 1. What is a 4-month-old baby spitting up milk? 4-month-old babies begin to enter the complementary feeding period. Since their digestion is not fully developed, they may vomit milk. This condition is often referred to as \”reflux\”, i.e.