Family Ties (1982)

S3E14 Oh Donna

Alex gets drawn into the life of Donna, a young, single mother-to-be from Elyse's natural childbirth class. His fascination quickly spirals into an obsession as he begins to envision a future for her child and behaves like they’re already a family. While Donna appreciates Alex’s attentiveness towards her and her baby, she soon starts to recognize that his eagerness to help is crossing into an uncomfortable territory.