Your employer is only required to hold your current position, ie a full time position if that is what you're doing prior to going on maternity leave. That said, a lot of employers can be very flexible with their arrangements.

I finished working in the last week of February 2005 & was in a full time job. I had Zander & then went back to work on Friday & Saturday only from the first week in September 2005. To this day, I am still doing only the two days (plus some work at home). I didn't discuss it with my employer until we were organising when I came back to work.