I doubt that there will be walkouts during the new film written and directed by Judd Apatow and likely to be a commercial hit. There is enough comedy and romance to give it good word of mouth. There is also far too much piled on and too many blatant improbabilities to support its romantic bent, but Apatow cleverly has his sites focused on what makes for popularity, as was the case with his earlier “The 40-Year-Old Virgin.” He also has a talented cast perfectly in tune with the material.

Ben Stone, played by Seth Rogan, is a good-natured slob of a California guy who has good-natured slobs for pals. They are slightly more grown up “Animal House”-type buddies. Alison Scott, portrayed by attractive Katherine Heigl, gets spaced out enough after partying to tumble into bed with Ben for a one-night stand. They are unlikely partners, and the incident quickly passes into memory until she finds she is pregnant and knows Ben has to be the father.

This is the sort of film in which abortion doesn’t even enter as a remote possibility. Ben, when informed, cottons onto the idea of a baby and is prepared to assume the responsibilities of fatherhood. The trouble is that he and Alison are absolutely mismatched. He’s seemingly a shiftless loser, she’s a go-getter about to become a television reporter. But we, of course, should know that the trajectory of the film will enable them to fall in love despite all the obstacles we can expect to be strewn in their way.

They don’t have very fine role models for domestic bliss in Alison’s older sister Debbie (Leslie Mann) and her husband Pete (Paul Rudd), squabbling parents. The comedy lies largely in Ben trying to adjust to the idea of an impending offspring while he does his best to help Alison prepare for motherhood. An example of the comedy is a sequence in which Alison hungers for sex and Ben fears intercourse will hurt the baby. They get into some explicit maneuvering that is likely to have those who adore the film in stitches.