First off I am sorry that you are having to deal with financial problems, I know how difficult that can be.
Secondly, some doctors will not take you off for maternity leave until around 35 weeks pregnant, unless there is something medically wrong. So unless you have something wrong going on with this pregnancy, the doctor still should allow you, and even at that, many moms work up right to when their water breaks.
Thirdly, I work in retail and though it was hard to deal with and cope with coming home with swollen feet and a sore back, it wasn't the worse thing in the world, I just made sure to take it easy the rest of the day.
Fourthly, I think no matter what kind of job you have, disability still pays you for Maternity leave. Look into it
:) Good luck!