If its not environmental, like Ruebella or CMV (illness contracted by mom while pregnant), its most likely genetic. My husband and I are hearing, no deafness in either side of the family, unless there was way way way back. My husband and I are carriers of a certain gene mutation. So any of ours kids have a 25% chance of being deaf; hence my 1 year old was born profound. But her having the gene doesn't make her kids more likely to be deaf, it just depends on the dad of her kids.
Its all based on chance, anyways. So even a genetically deaf person can have a hearing baby. And just because the gene is passed on, doesn't mean it will "show". Interesting stuff to be sure.
But I agree, my baby girl is as healthy as can be. Got great Apgar scores when she was born. Happy, carefree, smart, amazing.