She does definitely want a child, basically now, and has been pressuring me to get engaged to her. More worrying is that she says she cannot use medical birth control, and hates condoms and wants to rely on withdrawal and 'luck' :S Truth be told I am very co-dependent and when it came to talk of marriage and moving in I think it was even me that came up with the idea.
"Withdrawal" is not a form of birth control. . .and "Luck?". You are living dangerously. You need to delve further into her claim to NOT be able to use "medical birth control". There are a lot of options to choose from. Easy to understand that someone has had a problem with some form of birth control in the past, but highly unusual to have a problem with all of them? Could it be that she just wants to get pregnant at any cost? Best to validate the situation. How will you prevent pregnancy, after the first child (just keep leaving it to chance?) If she can't be totally honest about birth control, and have an honest discussion, then that is what you will have to look forward to with other issues.
How did she NOT get pregnant in the past, with her sexual history (and lack of birth control and protection)? Some women want to get pregnant, so that a man will marry them. If she won't allow condoms to be used, then how many STD's has she had? A promiscuous partner (who cheats), and won't use condoms or birth control, could well bring you an STD and someone else's baby.
Just a heads up. Depending on what antidepressant she is taking, it could be contraindicated for use during pregnancy (or at least at certain stages) It is something that needs to be evaluated. Hormonal fluctuations during/after pregnancy can have an impact on someone with depression and/or anxiety (commonly comorbid with BPD). What appears as emotional stability now, could change negatively during pregnancy.
The idea that she would regularly cheat and then divorce me and take my money is a huge worry.
Unfortunately, many people run into huge legal fees and financial loss, when they have to deal with divorce and child custody battles when they split up with a BPD partner
Her history includes abandonment as a child by her father who left her mother with no money, who has remained single for 27 years. She has tried to kill herself around 10 times. And the rape 2 years ago.
Her emotions are still unstable now and she will occasionally flit between being apologetic and then blaming me.
Did you partner have 10 suicide attempts, or was it her mother?
The blaming behavior is a typical BPD behavior. Is that something you can live with? It will likely continue.
Because I now almost know too much about sociopaths and BPD I can't help but feel every one of her actions and traits can be traced to something nefarious and I'm finding it difficult to know whether or not the person I love is real.
Step back and evaluate your situation (and use something to prevent a pregnancy while you evaluate). Do your homework on birth control menthods. Then, check your understanding on what specific methods she has tried and what the problems were. Find out which antidepressant she is on and do some research on possible issues with using it during pregnancy. A baby can't make things better. It will make things more challenging. The thought of a baby appeals to her, but the reality of a baby might escalate BPD behaviors.
It could be well worth your time and money to seek some individual counseling at this time. You say you are co-dependent. A co-dependent person commonly pairs with a BPD person for a dysfunctional relationship. The co-dependent person then serves the role of caretaker and rescuer. Is that what you want out of life? Can you see yourself growing old with your partner?
Now is the time to ask yourself the hard questions?  :)on't be passive and just let her get pregnant and then marry her, because she is pregnant. Not using any form of birth control or protection is passively agreeing with her plan to get pregnant and marry.
Best to start learning some skills that can improve communications with your BPD partner. It could be helpful for you to review the communication lessons/tutorials. There are several in the right-hand margin. There is, also, a "lessons" thread, at the top of the thread lineup. The large green banner, at the top of the page has a "Tools Menu", as well as other links to helpful information. The communication skills can make it easier for you by improving the way you interact with your partner and the way you react to her.
I know you have a lot to consider. A co-dependent person generally will just go with the flow. I hope you choose to set out of co-dependent habits and evaluate your situation and options. Take the time needed and the steps needed to consider what you future (and child rearing) will be like with your partner.