The couple have been together for four years but broke up after Stewart's affair with her married "Snow White and the Huntsman" director Rupert Sanders, reported Showbizspy.
"They had been talking about having a baby together once 'Twilight' ended, which is what makes this so extra-painful. Kristen knew how badly Robert wanted to start a family, and it was a subject that had dominated their conversations in recent months," a source said.
"Robert would be a great father, and having a baby would probably have brought them even closer together," added the source.
Although the couple has not released a statement on the future of their relationship, Stewart, 22, fears there is no way back for them.
"If Rob takes her back at this point it will be a miracle, but she knows she only has herself to blame," said the source.
Stewart admitted she had cheated on Pattinson, 26, with Sanders after pictures emerged of them kissing in a car.
Sanders has two children, Skyla, seven, and five-year-old Tennyson, with wife Liberty Ross.