Cut hours a month before maternity leave.

The organization I work for is going through cutbacks. They have decided to cut my hours to 32hrs/wk just a month before I go on maternity leave. This means that my Short Term Disabilty payout will be 60% of 32hrs/wk instead of 60% of 40hrs/wk. Is it legal for them to base my payout on the last month's pay rate, rather than the previous year's pay rate?
I had already areed to return to work at 20/hrs a week after maternity leave, so them cutting my hours a month before maternity leave seems very silly.