Last night I found out.
I was nursing her when she looked up at me and in her sweet little voice said, "da-da."
I was so excited for my husband since she had only made the "ma" sound so far. I called him quickly at work and this is the conversation we had:
Me: You'll never believe what your daughter just said.
Him (completely monotone): What do you think I do with her all day? We worked on that for an hour and half today.
Me (after a moment): Well it doesn't count because she's not saying it in context. She called me da-da.
We often have duels with our precious Peanut, trying to influence her to say "ma-ma" or "da-da" first each of us saying our respective name to her while she giggles.
So now I know he's been practicing with her on the sly and I need to step my game up. I think I'm playing at an unfair disadvantage. As I repeated "ma-ma, ma-ma, ma-ma" to her over and over again, she just keeps smiling and saying "da-da."