I wonder that about my own DH frequently. We have a 2 yo and one on the way as well, yet he finds it necessary to pay for things that are not his responsibility. You would think these grown men would stop GIVING their money away to able-bodied adult women who should be responsible for themselves. Unfortunately, I don't think I have any advice. I'm still trying to figure out my own DH and why he does this...

Well if they were married at the time this debt incurred I would have assumed they would most likely both be responsible for it.
Maybe your DH just wants to get it done and over with, and knows that asking BM for some help in paying it won't amount to anything.
I can see where your fustration is, but I don't see how she is getting away with anything. If he has already paid the debt down to 1200 then I would just make sure he continues to pay it until there is nothing left. I'm sure he didn't tell you she was on it because then you would expect her to be held accountable for it as well but it looks like he want to take care of it himself.
The BM in my sitch left my FH. She is the one that wanted the seperation/divorce so she could see someone else. Yet, FH was the one who foot the bill for the divorce.Why? Because there was always a reason why she didn't have the money, or why he had to wait. It just didn't make sense. So when he had the money he went and did it. It wasn't worth the aggravation to wait around for her to be ready.
I get why your upset. And this economy its tough. But try to understand that this debt needs to get paid ASAP and it probably won't be her that does it.

I understand the frustration...but you know what. You are expecting...you get paid less...so pull back on finances and tell your dh, since we are expecting i need to save money for our baby. I know you are paying cs...but think about getting a second job? Since you are trying to pay off your debt , your exwifes debt too and you pay cs, i earn less than you and we are expecting...so i've decided unfortunatley i cant' cover the half you expect because youare paying ex's debt...so please get a second job.

I earn more than my dh, he pulled the CS payment card on me and said oh...i should pay more of the bills cause he pays CS...What did i say...mmm
1. I didnt' F your ex to have kids.
2. I didn't divorce her to pay CS
3. I made it clear from the beginning that our finances are separate and CS and any payment concerning your kids, ie gas bills etc..is soley your responsibility.
4. I am not your personal bank account.

I can see how you would be frustrated! We too, have a baby on the way and are paying off debt from the previous marriage but at least DH did not lie to me. And yes, if my DH had lied to me about paying off BM's portion of debt, I would be FURIOUS.

It still eats me up inside just knowing that I am paying because of her, but I love my DH and I'm here to support him. I know that for my DH, he doesn't see it as taking care of BM. She would never pay her half on her own accord, so he sees paying the whole thing as taking care of himself and his new family. Getting the debt paid off is important for getting his badly damaged credit score back on track, and we can't even think of having our own home until that is repaired. I have even put $$ from my savings into helping him pay off some stuff, since it could speed up the time it takes to get us out of this hole.

Also, the "I lied because I knew you would get mad" seems to be a normal thing, it happened to me too for different things... and I was always MORE mad at the fact that he lied rather than the thing itself. Once I started simmering down my reaction to anything he had to say regarding BM, he started to be more upfront and honest.

Dunno if this helps, but hang in there!!