I hear this complaint frequently from new parents. There are a few things going on here. For the first few months after the birth of the child, her body is going to be all over the place with regards to hormones. Some women return to feeling sexual quicker than other women. Until she levels out, she may feel disinclined to be sexual. The other issue is now that she has to reconcile both being a mother while still being a sexual person. This can be difficult for some women. Birth also changes a woman's body, which can leaver her feeling less attractive or just not herself. Further, her responsibilities (and yours, hopefully) have increased and been altered in terms of importance. That’s not to say you aren’t important; I am sure she finds you to be very important. Now her priorities also include providing for this small, defenseless, co-dependent being the two of you created.
Have you expressed to her how the lack of time spent as a couple is effecting you? Not by simply asking for sex, but expressing to your girl that you miss time with her as a couple? Have you asked her how she feels about returning to the sexual parts of your relationship?
One way to begin getting intimate again might be to go out on a date, and to gently suggest that the two of you could work on re-introducing intimacy into your lives. Find ways to meet her halfway on this issue as well. Are you expecting only penetrative sex, or are you willing to try alternative ways to work back up to traditional sex? If her body is still healing, perhaps you two could try sensate focus as an option. Essentially, sensate focus begins by the two of you being together and touching each other in areas other than genitals/traditional sex spots and working back over time to penetrative sex. (Get more tips in I feel like I've lost my sex drive. What should I do?)
If stress and fatigue are an issue for your partner, you could also try offering to help her around the house or with the baby to reduce the load she carries so that she can create more time for you. You could also hire a babysitter to give you both some stress-free alone time.
If this has been going on for awhile, and you are past the initial healing phase, then maybe it is time to suggest she also return to her doctor. There could be biological concerns that need to be addressed. Maybe her hormone levels haven’t returned to "normal". She may also need to see someone about postpartum depression, or even just talk to someone about how to balance her new role in life with her relationship.