No I don;t think you're being pushy. For one thing had you not asked they might have made assumptions because you were pg.I don;t know how big an organisation it is, but if its a certain size they may have recruitment policies to adhere to. ie now that the job has become permanently available they may choose to/their policies may say they have to advertise it. ask for a meeting with your boss when he comes back. also don't phrase it in terms of "taking you back after 6 months" phrase it in terms of considering you for the permanent post. Also try to point out that the time and effort they'l spend on recruiting someone else, plu waiting for them to work their notice, plus training them up/settling them in - they'd be far better off just having you even with a 6 month gap.