This passage of Scripture could be very confusing unless you understand what David was talking about. This Psalm was written by David. Nathan the Prophet went to David after he had gone into Bathsheba; then David wrote this Psalm, and he repeatedly spoke of his own sin. He never mentioned Bathsheba, nor did he mention Adam's sin; he only spoke of his own transgression. David said, "Blot out my transgressions...Against thee, thee only, have I sinned." Wouldn't it be wonderful if everybody would own their transgressions? What a revival there would be if people would truly come clean and admit to their own sins!
David should have been out on the battlefield, but he sent Joab in his place. He tarried at Jerusalem, and he went up on the housetop. While he was there, he saw a beautiful woman bathing herself, and he was drawn away of his own lustful desires. James, the first chapter, tells us that every man is tempted when he is drawn away of his own lust and enticed, and when lust has conceived, it brings forth death. When did David's lust conceive? When he sent for Bathsheba, and it brought forth spiritual death to him.
In this Psalm, David said, "Behold, I was shapen in iniquity; and in sin did my mother conceive me." What does the word shapen mean? According to Strong's Concordance, it means "to fall grievously, with pain." In verse 14 David said, "Deliver me from bloodguiltiness, O God." David did not stand when the temptation came; he fell grievously, and it pained his heart that he fell. So to say the he was shapen in iniquity or that he fell grievously with pain is saying the same thing. No doubt when he thought about his having committed adultery and having Uriah killed, it pained his heart that he fell grievously in iniquity.
What about David saying, "In sin did my mother conceive me"? The word sin, as it is used in that particular Scripture, comes from the word chet, meaning, "fault or error." What he was actually saying was, "In error did my mother conceive me" or "I fell grievously in sin, and it was a big error that I was ever born." This verse in Psalm 51 is one of the most frequently use verses to prop up the inbred sin theory. Study it in the light of Job, the third chapter, and you will see the comparison. David's sin hurt him so much and he felt so badly over it that he wished he had not been born. I know this may be controversial, but I challenge you to study it. Look up those words in Strong's Concordance, and study them in the light of their true meaning. If you will, you will find that it will shed a different light on that Scripture.