Continuity mistake: When Allison is swimming with her nieces and Ben, she is sitting at the edge of the pool and you can see her stomach is normal and toned, but in the scene prior she meets with the gynecologist and you can see she has a fairly big belly. (00:54:45)
Continuity mistake: When Alison tells Ben she's pregnant and he starts talking about how she didn't notice he didn't have the condom on, the camera is on Ben and we see her hand going up to her face. In the next shot a split second later, her hands are both down in her lap.
Continuity mistake: When Alison and Ben are having breakfast in the diner after their first night together, Alison's hairstyle changes throughout the scene. (00:18:50)
Revealing mistake: Right after Allison gives birth and they are handing her the baby, you can see the baby's belly button, and the umbilical cord is below that.
Continuity mistake: During the doctor's office fight Ben tells Allison it's a girl. Later, in the tub Allison refers to the baby as a boy. Stating "then he'll be wired for stress for the rest of his life." (01:24:03 - 01:50:44)
Continuity mistake: After the earthquake, when Ben and Allison are sifting through Ben's stuff that's all over the floor, Allison grabs the baby books and sees that Ben has not read them. The first shot showing the books has them in order of "What to Expect When You're Expecting", then "Expectant Father". In the shot immediately after that, the books have been switched, and the shot after that, the books are back to the first order.