Job, I don't know what to do.

I babysit a couple kids, usually I only watch them 12-20 hours a week... 
They're really difficult and stress me out.  Now that I'm pregnant it's even worse.  They literally stress me to the point where I'm having cramps. 
I'm about to be watching them for like 40 hours a week, until almost midnight. 
I am exhausted and have nonstop extreme nausea... I don't think I can watch them anymore. 
I feel bad because I'm friends with their mom, and I need the money (even though it's hardly any) but I don't want to risk hurting my baby :( 
I already told their mom I wouldn't be able to watch them the whole 40 hours, but she didn't even listen to me :( 
What would yall do??