Often, when a new mother is suffering with PPD, she has a great support system of family, friends, and professionals. This is ideal. However, sometimes her partner is left feeling out in the cold. He suddenly has this new partner that he doesn't recognize or understand. He can wonder, "will I ever have my wife back?" We all expect that a new baby will bring certain changes. Often, though, we are unprepared for the actual changes that come with pregnancy and childbirth.
It is common for a man to feel that he needs to help, and wants to fix the problem. While this is a normal reaction, it is important to remember that this is not something he is capable of fixing. The best thing he can do is to offer support. Common mistakes that a partner can make in this situation are to expect his wife to just "snap out of it," to tell her she has so much to be thankful for, why is she so upset? Or to tell her things like, "you know what your problem is...?"- these kinds of statements imply that his partner has control over how she is feeling, which people who are clinically depressed do not, and they also imply that the depression is somehow her fault, which of course, it is not.
The best thing a partner can do in this situation is to just support his wife and their new baby. Try not to have any expectations. Try to understand that she is not feeling like this on purpose, and there is nothing that can be done in the immediate future to bring her out of it. Hopefully, she has a therapist that she is seeing, and perhaps as her partner, you could also see the same therapist, who could help to provide some understanding of how the new mother is feeling.
Understand that she may not be getting the rest that she needs, so any time that the mother has opportunity for rest or sleep, she should be taking it. Work around the house can either wait for later, or be done by the husband or someone else. Cooking is not something that she may be capable of, so ordering out for a while may be a solution. Shopping can be done by the partner or family members.
The biggest thing that a partner needs to remember is to be patient. This is a temporary situation. Eventually, with help from friends and family and professionals (and perhaps medication), he will begin to recognize his partner again. PPD is not a permanent condition. Patience, support, acceptance, and love will go a long way toward helping a new mother heal from depression.