For at least two generations, the pregnancy guidebook “What to Expect When You’re Expecting” has, informed, educated and, frankly, terrified parents-to-be. So who do you call when you want to turn that book into a rom-com? Jennifer Lopez, you’re wanted in the maternity ward. The actress-singer is a vet of more than 20 movies, with a solid third of them being feel-good chick flicks like “The Wedding Planner,” “Maid in Manhattan” and “Shall We Dance.” If anyone could help turn “Expecting” into a date-night destination movie, it’s the 42-year-old Bronx native and “American Idol” judge. “I’ve always loved romantic comedies,” Lopez said recently while promoting the new film, directed by Kirk Jones. “I’m a bit of a romantic, and — I don’t know if you’ve noticed — I am a little bit funny! I’ve just always been a fan of the genre.” In the film, Lopez’s baby photographer character Holly and her hunky husband Alex (Rodrigo Santoro), in one of five story lines, decide adoption is the best option for them. Lopez essentially anchors the movie, offering a way station between the burping, baby-bump-filled sections. (Cameron Diaz, Elizabeth Banks, Anna Kendrick and Brooklyn Decker round out the cast, in more ways than one.) In real life, Lopez carried her and ex-husband Marc Anthony’s now-4-year-old twins Max and Emme to term. “I think Jennifer was drawn to the role because it was so different than what she had personally experienced,” Jones says. “She was lucky enough to have been able to have children naturally, and she was interested in getting into the head of a character whose [circumstances were different].” Jones says that all of his actresses took on parts that stretched their comfort levels in one way or another. “Elizabeth Banks, who has children through a surrogate, was taking on a role where she would experience all the physical conditions of pregnancy,” Jones continues. “Cameron doesn’t have children, but was also interested in going through that experience and having a prosthetic tummy strapped on her. “Being the father of three kids, I knew that pregnancy is full of absurd and humorous moments.” Read more: