Late to the game here, and I hope you're doing well right now.
I am a huge proponent of marriage and relationship counseling, even when your relationship seems good. (Which, yeah, is expensive. Believe me, I know.) In my opinion, the way to approach it is something like, "I love you, and I feel our relationship isn't going well right now. I want to make this work, because I love you, but I'm not sure where things are going wrong, or even if it's in my head. Will you accompany me to a relationship counselor?" In my opinion, you don't want to give any indication that you blame her in any way, because that's going to turn someone right off.
Other people are right; this could be postpartum depression, it could be your own hormonal fluctuations if you're currently on-cycle or PCT, general stresses of having a new kid, or a combination. Don't go into therapy trying to find blame; that's a quick way to ensure that it's not going to work.
Second thing, I've said this to many people in real life: look at Arthur Aron's "The Experimental Generation of Interpersonal Closeness" in Personality and Social Psychology Bulletin (1997). The gist of it is that there are 36 questions, and you should take turns answers the same question (i.e., so each person answers each of the 36 questions). Take time to ask questions about your partner's answers, and really pay attention to what they're saying. You don't want to give them push back (e.g. "Well, that's stupid, why would you do X, Y, Z?"), just acceptance. And your acceptance of their answers needs to be sincere. Questions start out very superficial, and become progressively more emotionally intimate. If you are feeling like your losing the connection with your wife, this can be a shortcut to helping rebuild that. I did this on my third date with someone; we've been married seven years now, and were married less than a year after we met. It's not a panacea, but it can really help.