Why make employers eat the costs? Just make government pay equal benefits to every mother for x months and don't force employers to keep the position open. This way actually everyone will have the same rights unlike now where how much you get depends on how good a job you managed to land even if you only worked there for a very short time.

There is incentive to re-hire anyway if you have open positions as you get someone already familiar with the work. Forcing employers to do so is a problem for smaller companies having many non-duplicated positions though.