I haven't seen the programme, bu the unfortunates in our society reach prison by a variety of circumstances, none of which are standard in every case, but certain similarities do crop up regularly.
It usually starts with bad, or more often no parenting to speak of. Children learn a sense of right and wrong from their families, and a lot grow up in criminal households, or without guidance at important times of their lives.
These people usually end up unable to really direct and control their lives - they live while certain things simply happen to them and they lack the skills to be able to look ahead and make sound judgements about what is right and wrong, either for them as individuals, or for those around them.
For young women, a need for love and affection can lead them into ongoing bad relationships with men who abuse them, or turn them out on the streets as addicts or prostitutes, often both - and from there it is a short step to pregnancy and / or prison.
I can guess than even not having seen the series, that some of my analysis will ring true with the young women that have ended up in prison.
Here's a worrying statistic for you? How many street prostitutes are either adicts themselves, or support a partner who is an addict? The answer is all of them - 100%.
Until we address parenting skills at nursery level, this issue will never be addressed properly.