After breaking it off with her boyfriend, Billie has a one-night-stand with a younger guy. After ending up pregnant, Billie decides to keep the baby, as well as the father.
While snooping through Zack's stuff, Billie finds a note he was writing to her that he never finished. After finding this, Billie sets out to figure out what Zack is hiding from her.
Billie's pregnancy begins to make her more attractive and Zack takes notice of her new curves. However, when Billie has an incident involving self-tanning lotion, Zack is turned off by her appearance.
Billie and Zack agree to see other people. However, this proves to be easier said than done when Billie's pregnancy gets in the way of both of their potential dates.
Billie gives Zack and Davis relationship advice so he can live vicariously through them.
Zack tries to surprise Billy by installing a low-flow toilet in the apartment, but it ends up creating the first argument between them as a couple.
Billy asks Abby and Nick to be the Godparents to the baby, but it ends up causing them to break up. Now, Billie must stop Abby from going too far with Davis during a night out on the town.
Zack is jealous when Billie starts dating a co-worker.
James hires on a new film critic at the newspaper, making Billie feel that her job is at risk. In order to prove to James that he needs her, she sets out to write a better review than the new film critic.
Billie and Zack enroll in instructional parenting classes, but have to do it separately due to their conflicting schedules. Billie goes to class with Olivia and Abby, but Abby outdoes Billie at all the tasks, creating a rivalry during class. Davis goes with Zack to the classes where they meet two women.
Billie tries to find a way to hide the fact that Zack is living with her from her parents who come to visit for Christmas. While doing so, Billie worries that she'll ruin Christmas, as she has done in several previous years. Meanwhile, Abby's husband Nick tries to impress Abby's father.
In order to keep their surprise baby shower a surprise, Abby and Nick convince everyone to spend time with Billie and Zack, but their plan backfires.
While working on an article with Sully, Billie comes across some shocking news about the newspaper. Meanwhile, Zack and Davis decide to go into the sandwich business.
Billie, trying not to nag Zack, tries to trick him into doing chores around the apartment. However, her plan goes awry when Zack takes it upon himself to decorate the nursery. Meanwhile, Davis gets a job at a gay bar in order to earn some extra cash.
Billie invites herself to Zack's high school reunion, but with regrets when she becomes unsure if she can't compete with Zack's hot ex-girlfriend in her pregnant state. Meanwhile, Abby makes a miracle youth juice that she tries to sell at a party that she and Olivia host.
Zack is upset when Billie hires a baby nurse without consulting him. Meanwhile, Davis and Ryan are faced with a difficult choice when they learn surprising news about the nurse.
In the first part of a two-parter, Billie acts as if she gets into labor in order to save Zack from a traffic ticket of speeding. Meanwhile, Olivia and Davis spend the day shopping for a baby gift.
After faking labor pains to get Zack out of a speeding ticket, Billie goes into labor - for real. Unfortunately, things take a turn when Zack gets busted by the officer they had initially lied to and is hauled off to the police station.