1. Does a four-month-old baby need complementary food? A four-month-old baby has begun to develop an interest in solid food, but at this stage, breast milk or formula milk is still the baby\’s main source of nutrition. Therefore, it is generally not recommended to introduce complementary foods before four months. But if your doctor or pediatrician feels your baby is ready, single-ingredient solid foods can be introduced gradually. 2. How to judge whether the baby is ready to introduce complementary foods. Each child’s growth and development speed is different, so there is no unified standard to judge when to start.