I'm a cynical ass, so I'll take it with a grain of salt, but it certainly movie magic if it went down like that.

Here's the link:

http://www.nbcnews.com/entertainment/breaking-bad-producer-i-didnt-script-baby-saying-mama-8C11169986

And here's the pertinent quote:

You mentioned Baby Holly. She broke a lot of hearts and there's a campaign to get the actress nominated for an Emmy next year. We heard her first words, we saw her fear. How did you get her to do that?

You never know what you’re going to get with a tiny little person because beautiful little actresses of their age don’t really know what’s happening. And we have two baby Hollys and their moms are always right there. There are enormous restrictions when working with children that are appropriate in terms of you only get them for 20 minute increments. This is really emotional stuff. We really just kind of got lucky that the baby Hollys were giving us what we needed within a scene. Certainly, seeing Skyler pounding on the window of a pickup truck was really scary. But in the restroom scene with Bryan, her mommy was just three feet away. And I didn’t script the baby saying “Mama.” I scripted that Walter would come to it himself as he stood Baby Holly up and looked in her face and realized he was doing the wrong thing by her. And then, when he stood her up, and she looked over his shoulder at her 'Mama' and started asking for her, Bryan the consummate actor rolled with it. We just got some movie magic. It was just incredible. It was more than I could have hoped for — it was like the moment that I wrote on meth.