Learn English – “being” or “as being”

Generally speaking, it can mean the same thing - but it is not strictly equivalent, as deadrat pointed out. Peter Shor made a fair observation, through which i think OP asks for the antonym/lack of happy/happiness, in the given context.

Reading your comment about how you'd like to use it, bear in mind that unhappy (e.g. displeased and even sad, maybe), and not being happy (neutral, open to further exploration) are two different things.

Tread carefully; I'd advise using unhappy for displeased emotions. Using "not happy" is somewhat correct for this purpose, but not completely, as it can also mean neutrality, so don't do it. Or rephrase it - that could shed light on your intended meaning, and what you are trying to describe better.

The first two are grammatical and come across as idiomatic. The third does not.

In the case of Being 19 weeks outside the womb my baby has learned to grab things, you are stating that your baby is currently 19 weeks outside the womb and that, as a consequence, it has learned to grab things. Being 19 weeks outside the womb is a state that your baby currently is in.

In the case of Having been 19 weeks outside the womb my baby has learned to grab things, you are stating that your baby, at some time in the past, was 19 weeks outside the womb, and, as a consequence, your baby now has learned to grab things.

I hope that clears things up!