Corrected entry: When Sylvia is waiting in the hallway for the guys to come back from the airport, she is sitting on the floor with the baby. You can see the stroller next to her, and it is already fitted with the extender so all four of them can push the stroller together. They use this for the first time in the very last scene of the movie, after Sylvia and the baby move in. It wasn't used before.
Corrected entry: When Jack comes home from Turkey, and Peter and Michael are telling him about Mary, they read him the note that Sylvia wrote to him. After thinking he realises who Sylvia is and recalls that they got together a year and a half ago. That is 18 months. Assuming that Sylvia was pregnant the customary 9 months, that would mean that Mary should be about 9 months old. But later, when Peter, Michael, and Jack are going to the airport to catch Sylvia and Mary, they tell the cab driver that Mary is only 7 months old.