What Flowery said.
A zero hours contract is for someone whose hours are so unpredictable that you cannot put any set hours in the contract. For example, you might have a situation where a member of bar staff was on a zero hours contract because she had no set shifts and was only called in for holiday and sickness cover.
Your contract can make it clear that your current additional hours are maternity leave cover and that you will revert to your old hours if and when that lady returns to work, if that's what they are worried about.