While it is true that arching can be seen in children with developmental issues, it’s important to realize that the assessment of development includes looking at many behaviors in different dimensions. For example, we evaluate language and social skills as well as behaviors that have to do with muscular skills. Children with significant developmental problems may lag behind in one or more areas, but rarely is just one very specific behavior the only one involved.
The arching you describe is sometimes seen in children who are ‘stiff’ in their overall muscle tone, as in cerebral palsy, for example. Your pediatrician checks for this during regular checkups, but you can always mention your concern so that it can addressed directly.
A more common situation where we see this arching behavior occurs in babies with significant reflux of food. As they get that “acid burn’ feeling in the backs of their throats, they tend to arch backwards in discomfort. Is this more likely to happen in your child with feeds? Is she growing well? Bring these issues up with your doctor during her 4-month check-up.