When you say "BPD," are you talking about "Bi-Polar Disorder," or "Borderline Personality Disorder?" Although, the two can exist within the same person, they are not to be confused with one another. Bi-Polar Disorder can be managed with behavior modification and, in some cases, medication. It takes work, but it's possible. Borderline Personality Disorder, on the other hand, cannot be treated with medication and is rarely (if ever) successfully managed - these people do not have a conscience or remorse for what they do to others, whether their victims are husbands, wives, siblings, parents, coworkers, or random strangers found on the internet.
It never, ever gets better, especially when one feels "trapped" in the relationship by having a child in common. I would recommend a complete physical exam and lab work to rule out STD, consulting an individual counselor (even if you have to do it on the sly), running a complete background check (credit history, criminal and civil history, etc.), and consulting a divorce attorney JUST to outline options for you and your child, should you choose to end the union. Then, develop a sound decision that is based upon the best interests of your unborn child FIRST, and yourself SECOND.
Your unborn baby did not have a choice as to whom his/her parents would be, or into what type of environment he/she would be born. Too many contemporary parents have abandoned this understanding somewhere along the way - the CHILD"S NEEDS COMES FIRST, simply because they cannot speak for themselves, advocate for themselves, or process the adult issues in their as-yet-undeveloped psyches. Whether it means sacrificing time, clubbing, material desires, money, text messaging, "Facebooking," sleep, etc., the child's needs should come first, and that includes taking precedence over the needs of both adults. In a "healthy" relationship, both parents will work together, but in such an environment as you're describing, this is not possible.
Without hesitation, I will say that these types of situations NEVER get better - waiting for "something to happen" to alter the situation is called, "denial." It is quite possible (and, likely) that your husband will pass on a sexually transmitted disease to you and your unborn child. Don't think that's possible? Oh, it is not only possible, but more likely than not, given that he's engaged in a number of encounters.
OFF TOPIC: I would suggest to all men and women who are contemplating a legally binding contract of marriage to pay the fees for civil, credit, and criminal background checks. Does this seem extreme? To some, it may. But, if a potential partner has nothing to hide, they won't object to a background check.
Best wishes to you - I'm sure that you'll make the best decisions for you and your baby.