Basically anyone that is born, is born of the world, so the act of being conceived brings brings the stain of original sin upon each and every person.
There is an interesting article (pdf) that goes through some of the thoughts of the early church fathers that relate to this subject. This is one of the more relevant portions of the article:
What had remained implied in his letter to Simplician, Augustine now
states outright, that each of us born into the world is born in guilt.
Furthermore, having come to see in his earlier writings that to
suppose the preexistence of souls introduces an insuperable dualism
into the person, Augustine at this point is convinced that the guilt
present at birth could not have been contracted by one’s personal
choice, and so it must be attributable to the first man. Finally, the
agent said to involve us in the first sin is concupiscence
Going down the path of this question, and doubting original sin was a major controversy 1500 years ago, but St. Augustine fought against it, and for an introduction to what the Pelagian controversy is and why it is still controversial today you can look at major parts (2) and (3) in this page:
Here is what Pelagius taught, from the FAQ above:
By this time he had several disciples in different areas. His teachings were identified as:
- Denial of original sin (i.e. depravity or corruption) inherited from
Adam. Each person is born as a new, free agent with the same powers
of choice and responsibilities as Adam.
- Denial of original guilt received from Adam's sin. Among other
things, this called into question the necessity of infant baptism,
since there was nothing an infant needed to be baptized for.
However, Pelagius himself held that infant baptism was good and
should continue, though not for the remission of infant sins.
- Affirmation of the ability of men to be free from sin. Consequently,
the denial of the necessity of God's working in order to accomplish
freedom from sin. The power is in us, even if God helps.