In the Canadian promo, Snow White goes into labor. As she’s delivering her baby, the lights flicker. This is an obvious parallel to 3×1 when Emma delivered Henry and the lights flickered. At the time, many people assumed it was Emma who made the lights flicker, seeing as she’s magical. However, I thought then it was Henry who made the lights flicker, suggesting Henry has magic, too.
Seeing as how this time the mother doesn’t have magic, I think it’s clear it’s this new baby who has magic. Glinda and Zelena pretty much already confirmed in 3×19 that Snow’s baby would be magical. Glinda said, “This time you hold not one pure heart but two. I sense powerful magic there.” Zelena could tell just by touching Snow’s pregnant belly that this unborn child would “do quite nicely” probably because even in utero it has magic.
"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy