Maternity leave and short term disability question?

Monika

My employer says that for the first 2 weeks it has to come from my vacation/sick time and the last 4 weeks comes from short term disability which will pay only 60%. They told me the other 40% HAS to come from the rest of my vacation/sick time. Does this sound normal to anyone else? Why am I’m being forced to make up the rest of the 40%? I’m upset because I wanted to use the rest of it to extend my maternity leave.