"I was thinking I could, like, have the baby and give it to somebody who totally needs it," says the 16-year-old central character in the new hit film Juno on finding herself unexpectedly pregnant after her first sexual experience. "Maybe they'll, like, canonise me for being totally selfless."

As many have noticed, Juno is only the latest in a recent spate of American films in which young women coping with unplanned pregnancies take the "totally selfless" route and keep their babies, including last summer's box office hit Knocked Up, and critically well-received indie films like Waitress and Bella.

However much I enjoyed some of these films (Juno, especially), I can't overlook a kind of disingenuousness, even casuistry, at their hearts. It has not gone unnoticed that abortion has been categorically ruled out by these films, as being antipathetic to feel-good comedy.

Apparently we can't imagine a sympathetic character terminating a pregnancy, let alone viewing it as a happy ending. But the brutal truth is that for millions of women each year, abortion is the happiest ending they can achieve in an unhappy situation. The female protagonists of Juno, Knocked Up, and Waitress share more similarities than being pregnant, unhappy about it, and yet mysteriously unwilling to have an abortion.

Although Juno is being hailed as the first of these films to even permit its heroine to say the word, let alone consider the option, the film still stacks the deck by sending Juno to a filthy clinic staffed by an insensitive, clueless goth teen receptionist. Of course she hightails it out of there. So would I – straight to an actual clinic, one that was hygienic and staffed by trained, compassionate medical professionals.

But Juno decides to give the baby up for adoption, supported all the way by her compassionate, intelligent father and stepmother. She then finds a supportive, compassionate, intelligent couple who want to adopt her baby. This couple, like Juno, are white, well-educated and middle-class. Unlike Juno's family, they are wealthy, but Juno's problem is never an economic one. The adoptive family will cheerfully pay her medical expenses; Juno can recoil in horror from the suggestion that she might sell her baby, and explain that she is, like, totally selfless - because her parents are supporting her.

The reality, of course, is rather less heartwarming, and not just because all mothers are not automatically selfless (sorry, Mom). Teen pregnancies in the US are on the rise for the first time since 1991; President Bush's abstinence-only "educational" programs have, amazingly, failed to stop teenagers from having sex – or from getting pregnant. It has succeeded only in making them surprised: Jamie Lynn Spears announced that she was "in complete and total shock" at becoming pregnant.

Otherwise she is unrepresentative: nearly two-thirds of unplanned pregnancies in the US end in termination, and women living in poverty are almost four times more likely to become pregnant unintentionally than women well above the poverty line. The incidence of unwanted pregnancies and abortions has been on the rise among poor women, and among black and Hispanic women, while simultaneously declining among white, affluent women – like those portrayed in Juno and Knocked Up.

Teenagers who keep their babies overwhelmingly do so in highly racialised conditions of poverty, addiction, and violence, not in happy suburban homes; and the ones who give them up for adoption are often forced to do so, by economic circumstances, or by distinctly unsupportive parents and boyfriends. It's a scenario rich with comic possibilities, isn't it?

No doubt this summer's hit romantic comedy will be the story of a young black crack addict who decides to keep her baby, while much hilarity ensues. Defenders of these films point out that they are comedies, and that abortion isn't funny.

True enough. But neither is unwanted pregnancy. According to these films, getting knocked up is a laughing matter if you're rich enough, and white enough, to be, like, totally selfless. Tell that to the women coping with unplanned pregnancies, including the rich, white ones, and see if they laugh.

The writer is a senior lecturer in American literature and culture at the University of East Anglia