I haven’t found anything specific in the literature, but I cannot think of a rationale for ceasing a sidelying at 4 months of age. I heard this at a course. I nursed all 4 of my own children until they were around 1 year of age, which means that they were in sidelying (each side) during feedings for 1 year. I also find that if a baby with an intact sensory system has the drive to resist a side lying feeding position, then they are likely no longer in need of that protective positioning. I did read the abstract to the systematic review that was done by Park, Pados and Thoyre, but since I didn’t have access to the full article, I can’t swear that a timeline was not referenced, but it wasn’t suggested in the abstract. If the parent is capable and the baby accepts, I will have the parents feed from both sides if they are capable and steady and the baby will accept it, being mindful of vision development and head shape. I am curious to know your thoughts on the matter.
For me it is all infant-guided. I let the infant tell us. There is no arbitrary age for anything really, it is based on developmental readiness for a change, less need for a support. I suggest parents trial a more typical feeding position when infant has been home for a month or so and has been feeding without stress in a sidelying position. If the infant feeds as well in the new position as he did in sidelying, then the infant is telling you he is ready for the new position. If he does not feed as well in the new position, he is telling you he is not quite ready for a change and still wants that developmental support provided by sidelying.
An exception would be if the sidelying position with specific L or R side down was prescribed post swallow study or post-ECMO or post-PDA ligation. Then the change should coincide with therapist guidance and, if indicated based on etiology, a repeat instrumental assessment of swallowing physiology.