Writer, director and producer Judd Apatow had planned to use footage of a real birth for the movie, but he was told that the baby would need a working permit before they filmed and that would be impossible to collect because the baby was not even born yet.
The awkward tension that you can sense between Seth Rogen and Katherine Heigl was real. Even though it has been almost 10 years since they made the movie, people still ask the two of them about their on and off-screen tension as well as the rumors that they hated working together.
In the opening credits, there is a roller coaster scene that they had to ride so many times, that all the guys threw up and Jay Baruchel had a complete panic attack while riding it. Baruchel is deathly afraid of roller coasters so he made a deal with Apatow that he would be naked onscreen for the earthquake scene if he could get out of having to ride the roller coaster. Apatow said absolutely not and made Jason Segel do the nude scene instead.
Because getting "knocked up" is a English expression, they had to find clever translations for the title in international markets. In Russia, they named the movie to A Little Bit Pregnant, in Brazil it was Slightly Pregnant, and Japan they called it One Night, Big Belly.