I was going to say similar to Organicem. It might depend on your industry and although I am yet to return to work, if they can't offer me my old position back, they have to offer me a similar kind of work using the same skills. So for example, if I was a manager before, they can't offer me a base grade position on the front desk - they would need to find something that was the equivalent of mangement (or at least using some of those skills). Can you check the policy of your organisation against the law?

ETA: I'm assuming you're returning to work after a period on maternity leave and not some other kind of extended leave. They can't discriminate against you in that case.