When Kristin calls from the hospital that she is about to deliver by induction, there's no explanation of how she hoodwinked the doctors into a pregnancy that she didn't have. How did she "deliver" if she was never pregnant? How did she even get admitted to the hospital? The writers blithely ignore a major plot hole (amongst many others).
When Kristin brought the baby home from the hospital, it would have been immediately obvious to an observer that the infant was multiple weeks old and not a newborn.