My wife wants to kill our newborn baby help!!?

My wife Nyx gave birth 3 weeks ago and things haven't been good. She's been really depressed and started cutting herself again, she's already been diagnosed with Epilepsy and chronic depression and is on medication for it. I took her to the doctor but she said that it was just post-natal depression and it'll pass. Since then things have gotten worse, she can't sleep, she cries constantly, she's become extremely paranoid and seems to hate our son. She keeps saying he's possessed by demons, reads her thoughts and most bizarrely of all thinks he's trying to kill her. Whenever I try to reassure her she snaps and accuses me of wanting to kill her too. I recently found her in the nursery standing by the cot with a knife and I've had to take time off work to watch her. I love my wife and I don't want to leave her but I'm worried that she might kill herself or hurt our son if this keeps on....somebody help!