This was my initial theory when I heard that Lizzy was pregnant. Saul found out and didn't know if it was his, you would think he would know. Scratch obviously is attached to the baby and said it would not be used against Lizzy. We also know that Angel was part of this notorious Bloodline and Scratch killed him. So I think to prevent herself from being punished by the family, she had Lizzy be a surrogate mother to Angel's baby. It would be scratch's bargaining chip against the family. I don't recall Lizzy ever mentioning whose baby it was and so I think she agreed to be a surrogate mother for her life.
It is kind of a reach, but if anyone knows anything to disprove my theory I'd love to hear it.