The Brady Bunch - Season 5 - Eps 17: Welcome Aboard
Carol breaks the news to Mike that she's expecting and an eavesdropping Bobby and Cindy assume that she's pregnant and pass it along to the rest of the family. Unknown to them what she meant was that she's expecting her nephew, Oliver to move in with them over the summer while his parents are in Africa.
About The Brady Bunch

Title: The Brady Bunch
First Air Date: 1969-09-26
Last Air Date: 1974-03-08
Status: Ended
Rating: 6.719/10 (from 146 votes)
Language: EN
Seasons: 6
Total Episodes: 117
Network: ABC
Genres: Comedy
Production Companies: Paramount Television
Synopsis
When widower Mike Brady marries a lovely lady widow Carol Ann, their two families become one. These are the misadventures of this new couple, their six children, a dog named Tiger, and quirky housekeeper Alice.
Cast

Robert Reed
Mike Brady

Florence Henderson
Carol Brady

Ann B. Davis
Alice Nelson

Maureen McCormick
Marcia Brady

Eve Plumb
Jan Brady

Susan Olsen
Cindy Brady

Barry Williams
Greg Brady

Christopher Knight
Peter Brady

Mike Lookinland
Bobby Brady

Robbie Rist
Oliver