The baby books I read as a young parent told me that their first words would be ones that were easiest to say. Starting with a hard consonant. Probably a D. Don't be offended Mama, if they begin with Dada. It's just linguistics. It's not personal.
Newer studies, though, show that the nature of first words vary by culture . Here in the U.S., children are far more likely to start with a common noun than children in Beijing; and Chinese children are much more likely to begin with a name of a person or with a verb than are Americans.
Which means we can bring more to first words than just ease of pronunciation. They are infused with context, and worldview. The reason they thrill us as parents is that they are our first glimpse of what is going on in that little noggin.
I think it is really fascinating that this varies by culture! Has baby spoke his/her first word yet? If so, what was it?