So what is the child doing motorically at 3 to 6 months during the period of Babbling?
This quickly gives way to a modified consonant-vowel kind of cooing interspersed with nasal and nasal and labial fricatives.
Later it begins to carry some social significance and is used as a tool for gaining attention—a very positive sign for the development of aural communication. There will be identifiable combinations as well as one syllable utterances.
At the start of the babbling period, most notable motor behavior is the ability of the baby to hold his/her head up from a prone position (lying on his stomach). My feeling is that if you have been letting the baby roll and play a lot on the floor that he/she will gain this ability a little earlier.
It’s a desirable skill so that the baby can focus on more objects.
By half way through the babbling period, the baby should be able to sit with support; and by the end of the period, sit without support.