Where? I would like to know.
When you read the story of Jesus' birth you will find that Mary was a completely normal person. Look at Lk 1:29: "Mary was greatly troubled at his words and wondered what kind of greeting this might be". Apparently she had never met an angel before; nowhere in the whole Bible is any indication that she's had an encounter with God before or that God had removed her original sin in any way. Quite the opposite: the angel says "you have found favor with God". He doesn't say "you
had found favor with God [and He made you special years ago by removing your original sin]", no, his words indicate that the whole plan of God using Mary starts only
right now. Mary was an "average" young girl, if I may say so, and only at this point her life changed, when the angel announced that she'll receive a child. No word about her being holy or pure, no word about her not having original sin because she was born in a natural way herself. You find the same in Mary's own words (Lk 1:46-48) when she acknowledges that she actually isn't anything special but God uses her
nevertheless.
And this is what you see throughout the entire Bible: God uses the normal, average people! Those who are nothing special.
Lastly there is also the logical argument for Mary having original sin. Why did God have to become a human being to die for our sins? Because He wanted to? No, but because it had to be a sinless person! If Mary was holy, without original sin, then God didn't have to die. Mary could have died for our sins then. Jesus only had to be born and die for us because there is
not a single sinless human on this earth. Why not a single one? Because all humans have original sin. If Mary didn't have it then Jesus' whole mission was needless.
The bible does not support that conclusion as Jesus was Mary's first child.
How does it matter whether it was the first or not-first child?
Yes that is true that God the spirit does not have a mother but Mary was indeed the mother of Jesus.
She was the one carrying out the baby, yes. That's pretty much all the Bible says about her. How does it contradict the conclusion of Mary being a surrogate mother?