Jodi (Charlyne Yi) is supposed to be stoned in every scene she's in. However, in 2009, Yi told Tonight Show host Conan O'Brien that she had never gotten high before, or after playing the role, and in fact didn't even know that the character was supposed to be a stoner at the start of her audition.
According to the DVD commentary, Jay Baruchel made a deal with Judd Apatow that he would be the naked guy in the earthquake scene, if he didn't have to ride the rollercoaster, because he is terrified of them. Apatow broke this deal by making Baruchel ride, so the part of the naked guy went to Jason Segel. Baruchel's frightened reaction in the rollercoaster scene is real.
Judd Apatow has stated on the DVD commentary numerous times, that the film is partially based on the birth of his and Leslie Mann's first daughter, Maude. Scenes taken directly from reality include: - Ben trying to get their doctor on the phone, and cussing out numerous doctors when he can't. - The cantankerous staff physician whom Ben has to take out into the hallway to calm down. - The argument on the way to the clinic, where Allison throws Ben out of the car, and then they argue at the clinic when he gets there. And the scene where Allison tries to take a bath to calm herself, just before going to the hospital.
"Knocked up" doesn't mean anything in most languages. The film's translated Russia title is "A Little Bit Pregnant." In Brazil it's "Slightly Pregnant." In Italy it's "Very Pregnant" ('Molto Incinta').
Judd Apatow was originally planning on using real footage of a woman giving birth for the delivery scene. However, at the last minute, he was unable to because he needed a worker's permit for the yet-to-be-born child.
Paul Rudd contributed much to the discussions and arguments between Pete and Debbie in the film, based on real-life interactions with his wife. The character was loosely autobiographical for Judd Apatow to begin with.
In one scene, Ben (Seth Rogen) mentions that he likes Munich (2005) because "Jews kick ass." This was improvised by Rogen. However, the final film uses only one-third of the complete improvisation (not included in the DVD special features). After the film was released, Steven Spielberg phoned Judd Apatow to compliment the movie, and Apatow sent him the complete uncut improvisation of the scene. Rogen would later co-star with Munich's Eric Bana in Apatow's Funny People (2009).
The scene where Ben (Seth Rogen) dances with Allison takes place at a Club in Los Angeles called Plan B. Coincidentally (given this movie's unwanted pregnancy plot), "Plan B" is a brand name for the progestin-only emergency contraception Levonorgestrel.
The crew members in the studio scenes at E! are actual members of the E! News (1991) crew. However, the editing bay where Alison edits her pieces with Brent, isn't an editing bay. It is the studio control room for E! News and the Daily 10.
Jay Baruchel is terrified of roller coasters. In the opening sequence, where they are at the amusement park, he was forced to go on it by Judd Apatow. He only agreed to ride it once, whereas the rest of the cast rode the coaster seven more times, to get all the right takes.
The montage scenes where we see Seth Rogen and his friends ride a rollercoaster proved difficult. Multiple takes meant that Rogen vomited frequently, while Jay Baruchel has a pathological fear of roller coasters and took a lot of persuasion to go on one in the first place.
Although the role of Ben Stone was always meant for Seth Rogen, Judd Apatow created a mockumentary for the DVD titled "Finding Ben Stone". The fake documentary chronicles the fictional hard time that Apatow had in finding the right male lead for his film. Among those featured in Finding Ben Stone are Michael Cera, James Franco, and even Orlando Bloom. Although the documentary is clearly satirical, many have taken it to be true.
David Krumholtz, who reportedly is a very good friend of Seth Rogen, was supposed to play a character named "Gummy", but backed out, because he was offered the lead role in a Woody Allen film. However, the Allen film fell through, and he didn't get the part for "Gummy" back. There's an interview with him and Judd Apatow on the DVD Special Features talking about this.
Katherine Heigl has said in several interviews in 2008 and 2009 that she considered the movie to be sexist, because it depicts women as "shrews", and men as "lovable", and commented that she found the movie "hard to love". Seth Rogen and Judd Apatow were unpleasantly surprised to hear it, as they remembered working quite well with her, and quickly responded by saying that Heigl's The Ugly Truth (2009) didn't make women look any better. In 2016, Heigl apologized to Rogen and Apatow, saying that her earlier statements reflected the criticism of many women who found the movie sexist, and calling her own comments "immature". Rogen commented on this that despite being displeased by the remarks, he never held any hard feelings towards her.
As a special feature on the DVD, there is a fake scene were Michael Cera gets fired from the film, due to arguments with Judd Apatow. There is also a fake documentary where the studio is apparently unhappy with Apatow's direction, causing them to appoint Bennett Miller as co-director, much to Apatow's annoyance.
The early scene in which Allison is driving Sadie and Charlotte to school, Charlotte says she wants to listen to Green Day. In the sequel This Is 40 (2012), Billie Joe Armstrong, frontman of Green Day, makes a cameo appearance.
Ben Stone, the name of Seth Rogen's character, is also the name of Michael J. Fox's character from Doc Hollywood (1991). In this movie, Ben and Pete discuss the Back to the Future franchise, in which Fox starred. Ben Stone is also the name of Michael Moriarty's character from the long-running television show Law & Order (1990).
In the scene where Allison and Debbie are trying to contact Ben after she finds out she's pregnant, they are in Pete's office. Since Pete is in the music industry, he has pictures of him with various celebrities, and right behind Debbie is a picture of Pete with Elvis Costello. This is actually a shot from the movie 200 Cigarettes (1999), in which are both Paul Rudd (Pete) and Elvis Costello. Costello makes a surprise appearance at the party towards the end of this movie.
The trivia item below may give away important plot points.
In the ending scene, when Ben tells Alison that he got them an apartment in East Los Angeles, they joke about wearing gang colors of the Bloods, Crips, and The Latin Kings. They would be extremely unlikely to run into any of the three gangs in East Los Angeles, since the Bloods and Crips are not well known to be active in that part of town, since it is predominately Hispanic geographically, while the Latin Kings are more known to occupy other major cities, such as New York City and Chicago, and do not have a known presence in East Los Angeles. But, that point is rendered moot, since Ben was making a wisecrack, and not a serious comment.