I suffered with depression after the DS and I freely admit to being a complete bitch to DH. The truth is I was totally overwhelmed with the massive responsbility of caring for a new baby and I really resented the fact that my DH still had a life. He still had an income, friends, a social life, an escape route if you like.
He tried to help so much, bless him, but everything he did sounded like a criticism to me. I was going crazy trying to create the perfect bottle washing/sterilising system and would become hysterical when DH did it 'his' way. He didn't understand the hours I'd put into perfecting my system and his 'refusal' (inconsiderate bastard to follow my method seemed like more criticism.
Of course, he could have done things differently sometimes, he was a classic fixer, which made me feel even more rubbish but I have to be honest and say he couldn't have made me happy at that point, I was ill.
I did get better OP, with anti depressants and CBT. The only advice I can give you is listen to your wife, sympathise, ask what you can do to help, encourage her to see get out and see friends. Try not to offer solutions, it can just reinforce the feeling of being a complete failure as a mother, try not to judge her and don't give up on her.