Sign the contract. Legally you're not required to advise your employer until the point in time that is specified in their mat leave policies (usually a month or two before you start leave, unless you need modifications to your work before then). So check the policy that applies to you as it may ease your mind.

In my experience staff are usually much much more loyal than employers. If the shoe was on the other foot & the company was thinking of terminating your position you can bet they would leave it until the very last moment before advising you. And if you're feeling vulnerable at work, consider joining the relevant union where you can get advice & support re issues like these.