I am being let go from my position at a temp agency when I give birth, becaue they can't accomidate me taking a maternity leave. Is this legal?

I started working at the company as a full time employee over three years ago. I was even hired as a permanent temp before I found out I was pregnant. Once I realized I was pregnant I transferred into a part time position at the company due to how physically demanding my job is. The part time position is with the same exact company, doing the same exact job, just with less hours. I was told that when I give birth that my assignment will immediately be ended due to them not being able to accommodate me being gone for maternity leave. How is this fair when I have put so much time into this company with no complaints against me? And if I was still a working full time hours they would have had no problem giving me a maternity leave....... They did tell me that when I am ready to come back I could, but all of my seniority, and pay raises would be reset to as if I was a new hire. This doesn't seem fair to me.