Retained Moro, Or Startle Reflex
Moro Reflex is developed to help protect the baby from danger through the sensory system and taking their first breath of life. This is the "fight or flight" sense. You'll have seen this when a newborn is startled or receives a sensory input like a sudden light or sound, their arms will flair out. Then the baby takes a deep breath and curls up crossing both the arms and legs.
This should be integrated between 2 to 4 months. However, because of changing environments, procedures, and lack of tummy time, more children are retaining this reflex.
Test for Moro Reflex
Have the child sit on a low chair or lay on their back. Ask them to open their arms and legs out like a starfish. Now ask them to bring them in crossing them as they curl up. You may need to demonstrate it for them or let them see the pictures below. Generally they will cross with the opposite arm from leg on top. This is normal, at first, and the way they did it when startled as an infant.
Now ask them to spread arms and legs out again and cross/curl up, again but with the same arm as leg on top. Right leg and right arm on top. If they are too young to know right from left, put a sticker on the back of their right hand and on their right foreleg. Ask them to cross up with stickers on top. Now do the same with the left side.
If they struggle to do this then the reflex is still present.