Depends what the nagging is about / focused on -- if it is shit that you really ought to have bit more awareness of AND broad consensus exists to support the "nagger" over the "naggie" {doing the dishes, spending responsibly, being involved in child care, etc) I think that is one thing. More than slightly different is 'generalized nagging' of the sort that suggests dissatisfaction with a broad range of topics / behaviors, upon which there is decidedly less consensus as to there being a single "correct way to do it".
Good examples is this thread:
http://www.nairaland.com/nigeria/topic-168236.0.html (though the tables are turned and it seems to be an international forum...) Salty soup? Unwhite shirts? Something else is at work.
Here is the simple explanation:
http://www.todaysseniorsnetwork.com/Nagging_Spouse.htm 'Reactance' -- a fear of loss of autonomy. Makes perfect sense if new mom sees baby "consuming" all identity that she'd project that loss of individualness about dad and attempt to usurp some of your autonomy. Depending upon the degree to which this is something that the two of you can rationally discuss, the "traditional" course of talking about "I feel hurt when you criticism my ___________" might be enough, but even then some third party stuff (workbooks or counselor) can be helpful. The other thing, is that while I used to be pretty dismissive of vitamins and low doses of the popular prescriptions for depressiony kinds of things, I do think that they can be helpful. From PMS to just exhaustion there is A LOT going on when you go from happy-go-lucky rabbits to having a brood to deal with (even one)...
Good Luck! Voice of experience...