It was revealed in "HALLOWEEN HANGOVER" that she and Evan had begun a relationship. In "A Vinny/HYBRID Christmas" Steph was shown with an at home pregnancy test, presumably to determine if she was carrying Evan's child. The results of the test were not revealed at the time, but it was made clear in "Twenty-four months" that she had indeed been pregnant.
The child was presumed to have been born sometime around July or August, based on the timing of the pregnancy test, but there was no confirmation of that event until the child was shown being cradled by Evan/HABIT in "Next". In that video Steph can be seen in the foreground, while Evan is standing behind her, holding the baby, and talking to (it is assumed) Jeff.
Based on the memories shared by Evan in "WAKE UP", it appears that while possessed by HABIT he killed, and possibly ate, the child. Evan also reveals in that video that it had been a girl.