Is it harmful for a baby to lie down while breastfeeding?
Generally speaking, it has little impact for younger babies to drink milk while lying down. However, if a baby always drinks milk in this position, there may be some effects. The analysis is as follows:

Newborns and younger infants are usually fed while lying down, which generally causes no significant problems. Lying down to feed can timely provide nutrition to the baby when he or she is very tired or needs to rest. In such cases, ensuring the baby's head is slightly elevated can help reduce the risk of swallowing air and reflux, thereby decreasing occurrences of gas and spitting up milk.
However, if the baby always drinks milk while lying down, there may be some impact. Prolonged milk feeding in the lying position may prevent the baby's neck and spine muscles from receiving effective exercise, which could affect their normal development. As breast milk or formula contains sugars, babies may easily fall asleep while drinking milk in the lying position, causing milk to remain in the mouth for extended periods, increasing the risk of tooth decay.
After feeding, promptly hold the baby upright and pat his or her back to help expel excess gas from the stomach, thus reducing spitting up of milk.