Look folks I am not saying that every child who has been abused or neglected will carry on and parent in the same way. Some (like the ones TL mentions) will have seen a different model of parenting than the one they had, and will resolve to parent in a different way. I have seen cases like this in my work. One stands out - a brother and sister and the girl had been sexually abused for several years. I know this is hard to believe but the girl's mother had been sexually abused by her father (girl's grandfather) and went to prison and got friendly with another paedophile in prison and sugested he go and live with his daughter and her children, as he was released first. The father/grandfather would know full well what would happen. The mother was rather a timid sort who could not stand up to anyone (and she had been sexually abused by her father) and of course her girls were sexually abused by this man released from prison. The eldest girl was 17 when it came to light and left home and we did not know where she had gone. The other 2 (girl aged 13 and boy 11) came into care, with mother's consent - only the girl had been sexually abused, though the older sister (I assume) and the younger brother had had a miserable time with this man and a mother who failed to protect.
Anyway the brother and sister were placed with some foster carers on a short term basis and everything went so well, they were approved to care for them on a permanent basis. Both did well - the girl went into social work and the boy into engineering with the male foster carer's business. Those kids were brilliant anc a real credit to their carers.
I think TL you are being overly optimistic about the girl you quite rightly describe as needing parenting all over again. You mention that any baby she has will likely be removed quickly and adopted and this will brake the cycle. Thing is these children are not always picked up and never come to the attention of social services, health visitors etc. We have no idea of course how many children are in this position, but much more than we think I reckon.
Then it can take a long time to remove a child - first thing will be to put in support etc (like baby P) and an enormous amount of damage can be done by the parents in those early months (and sometimes years). Yes some will be more resilient than others and will come through unscathed, but many will not.
So really I am looking at this statistically. We know that children who have been in the care system are over represented in prisons and homeless people, addictions to drugs and alcohol etc. The study of sociology deals with the way in which patterns of behaviour occur in society (not individuals) because there will always be the exceptions. I remember reading a sociology book called "Born to Fail" showing a ragged little boy on the front which demonstrated by research how the cycle of deprivation is perpetuated.