Getting fired for taking 4 weeks maternity leave?

I have been in this hotel for 6 years 4 of wich i worked full time, then changed to 3 days a week. Im due next week and my manager just told me that i must sign a resignation letter cause im part time and they dont allow maternity leave. But how come other employees for example this guy who works there, hes only here maybe once a month or once every month and 1/2 so why isnt he fired? I have 60 hrs of paid vacation aculated from the begining of the year, so why cant they just put it as vacation leave instead of meternity leave?? this seems so unfair i dont understand and im getting so depressed!!! How is it possible to get fired for taking 4 weeks to care for my newborn and others not get fired for going on vacation for weeks or for getting sick????