I mean, clearly these two aren't the smartest, but I still think it's more likely that Savanna herself texted her mom. If not, there would be no reason for the couple to implicate themselves. They probably didn't even realize that Savanna told anyone where she was going.
ETA: This is my theory, obviously just my own opinion. They invite her up to the apartment thinking that no one would know where she was. They have a scuffle in the bathroom which the downstairs neighbors overhear. Then her brother shows up, they don't answer the door. Shortly after her father comes upstairs looking for her, they say she's not done. At this point, they realize that the family knows where she was going so they have to get her out of the apartment. She must have been alive at this point if the baby is only 2 days old so they somehow convinced her to leave with them. Possibly ensuring her safety if she goes with or threatening to kill her/the baby if she doesn't leave with them. At one point, there were reports that she was seen outside the apartment around 2:30. They move her to another location. The male probably stays with her while the female returns to the apartment. Around 4:30, Savanna's mom knocks on the door and the female says she has left already. That would explain why there was reportedly no forensic evidence of Savanna or the baby during the initial searches. And could very well be why the suspects felt comfortable letting LE search the premises. Maybe they planned to keep her until she delivered or maybe they just needed to gather things for a 'C-section' and that's what held things up a few days.
