It’s usually okay for a one-and-a-half month old to sleep flat, but it’s not recommended to lie flat immediately after a meal.
Sleeping flat on the back is the main form of sleep for small babies, and most babies sleep in this position. However, it is not recommended to lie flat immediately after feeding, you can hold the baby up against the shoulder to burp, and then you can elevate the upper body to lie flat, or choose the right side of the bed, to help improve the symptoms of gastroesophageal reflux in babies.
It helps to avoid the phenomenon of accidental aspiration or choking, and then change to the flat position after half an hour.
Usually must carefully observe the little baby, if there is any abnormal symptom, should go to the regular hospital in time to avoid adverse consequences.