Amvā (Amva) is a synonym for the mother who gives birth. Since the mother nurtures the limbs of the child, and help their growth, she is called Amvā —
kukṣisaṁdhāraṇād dhātrī jananājjananī smṛtā
aṅgānāṃ vardhanād amvā vīrasūtvena vīrasūḥ.