That's astonishing! I work in a very female friendly industry and this would never happen. People regularly work up till very near the end of their pregnancies and would never be treated any differently unless they needed to pull back, in which case they would be supported in that, not made to feel inadequate.
You need to say right now (nicely, of course) that you do not expect nor wish to be treated any differently. You could also reassure them that you will do everything you can to work with them to prepare for a smooth transition to whoever is covering you, but you certainly don't need to start 'handing over' now, for God's sake!
I took six months (couldn't afford and didn't want to take more), but most of the women I work with take 9-12, and are covered well for their leave and return to open arms. Really, this is the 21st century - incredible that you are being told to 'take it easy'.
My dd is two now, and yes, life has changed, but it certainly hasn't affected my career or prospects, and you do not have to put up with this.
Is this normal behaviour in your company? Do you know what the experience of other colleagues has been when they've taken maternity leave?