Psalms 51:5: A Sinner at Birth?

What does David mean by his being sinful at the time of his birth, indeed, from the time that his mother conceived him? Does he mean he was born out of wedlock, or that matrimony is evil, or is he teaching something else? How could David sin in the womb or at the time of his birth?

There is no hint here that David was born out of wedlock or that he had committed a particular sin as he was being born. His confession is that he is a sinner not only in act or deed, as his affair with Bathsheba painfully pointed out, but also by virtue of his nature. Original sin was present even before he was born and ever did even one act. David confesses that he had a sinful nature that must be confronted by God's righteousness and holiness.