When the "Cheaper by the Dozen" movies were made, neither Steve Martin nor Bonnie Hunt, who played the parents of 12 children, had ever had a child in real life. Steve Martin first became a father years later in Febuary of 2013 when his wife Anne Stringfield gave birth to his first child.
From oldest child to youngest child, the list of kids is: Nora (23), Charlie (17/18), Lorraine (14), Henry (12), Sarah (11), Jake (10), Mark (9), Jessica (7), Kim (7), Mike (6), Kyle (5), Nigel (5). In reality, Jacob Smith (Jake Baker) is 3 years older than Alyson Stoner, who plays his older sister Sarah Baker. As well, Morgan York (Kim Baker) is the same age as Alyson Stoner, and Forrest Landis, Blake Woodruff, and Liliana Mumy are all the same age when their characters are a year difference at least.
Steve Martin is a huge fan of the Doctor Who (1963) series, and playing a character named "Tom Baker" is the closest he'll ever get to accomplishing the goal of so many Doctor Who fans: playing The Doctor, a character who for seven years was played by actor Tom Baker.
After the meat soaking scene, Steve Martin's character (Tom Baker) says, "Where did they come up with soaking his underwear in meat?" Actually, the idea of the kids soaking Hank's underwear in meat was Steve Martin's idea.
The Gilbreth family on whom the film was based had 6 boys and 6 girls. Also, they did not have any twins. All the children were single births and the father regretted this because he felt multiple births to be more efficient.
At the beginning of the movie, Kate reveals the title of her new book to be "Cheaper By The Dozen". Although this movie is loosely based on a book by that title about a real-life family, the book was written by one of the sons, Frank B. Gilbreth Jr., along with his sister, Ernestine Gilbreth Carey. Although this film is based on that same book that inspired the 1950 film (Cheaper by the Dozen (1950)), the only element it retains from the original is the idea of there being 12 children in one family. Even the names of the characters are changed.