Here's a recap that explains it better than me:
Amanda’s pregnancy test, which she asks Emily to finesse if necessary thanks to a one-night stand that could’ve caused some trouble, comes back with Jack as the daddy. But to make the curly-haired mama-to-be feel terrible, Em lies that the baby actually isn’t Jack’s – a move Nolan calls “dark, even for you.”
Revenge ‘Resurrection’ Recap — Season 2: Hit Me With Your Best Shot - TVLine