I have a twentysomething friend in the US who, for purposes of preserving our friendship, I'll call Mary. As Mary is a human being, she likes to have sex. One thing Mary does not like, however, is contraception. Being on the pill made her "crazy"; getting an IUD felt, she says evocatively, "like having a hair caught in my throat"; and condoms "just don't feel good. We all know that." So for the past dozen years ("at least") Mary has been using an alternative method: she hasn't been using any contraception at all.

Instead, she has worked out a formula that she calls "amazing" and I call "voodoo". It involves a combination of relying on various smartphone apps with names like Period Tracker and relying on the guy she is sleeping with (she is not in a long-term relationship) to "behave" – in other words, pull out in the nick of time. That she has not become pregnant since switching to her voodoo system proves, she says, that it works, "although there have been a few plan B [morning-after pill] moments". Mary is not crazy. She is not even stupid. In fact, she is increasingly typical of her generation.

According to a report from the Guttmacher Institute, more than half of the unintended pregnancies in the US occur among the 10.7% of women who use no contraceptive method at all (and no, downloading Period Tracker does not count as a contraceptive method). This finding comes only a few months after a study carried out by the amazingly named Dr Annie Dude at Duke University. Dr Dude's findings revealed that 31% of young women in America aged between 15 and 24 had relied on the pull-out method at least once. Unsurprisingly, these women were 7.5% more likely to rely on emergency contraception than others and, even less surprisingly, of those who relied on the pull-out method, 21% had become pregnant. Apparently, these women had never heard the old joke: you know what you call a couple who use the rhythm and pull-out methods? Parents.

When researchers from the Guttmacher Institute asked the women who accidentally became pregnant why they eschewed contraception, answers ranged from the self-deluding ("a perceived invulnerability to pregnancy") to the predictable ("lack of thought or preparation", dislike of contraceptive methods) to the absolutely infuriating ("male partner's objections and fear that pregnancy prevention is an indication of infidelity").

Another factor I have noticed is that some young women resent having to shoulder the responsibility for contraception. Why, these women ask, and not unreasonably, are they the ones who have to take a hormonal pill every day, or have something stuck up inside them? Let the guy deal with it from now on! It's a remarkable turnaround since the 1960s and 70s when second-wave feminists argued that a woman's control over her fertility was a necessary power. Margaret Sanger, before she founded Planned Parenthood in America, wrote in 1920 that a woman who relies on a man for birth control is "exploited, driven and enslaved to his desires". Some young women today disagree and see having the control as a burdensome, irritatingly one-sided responsibility. One aspect that has undoubtedly played a considerable part in this shift is that many women have experienced negative side-effects from the pill, from making them feel – as Mary says – "crazy" to a loss of libido to a fear of blood clotting.

Talking of men, though, leads to a crucial point that these studies barely touch on: an accidental pregnancy almost certainly requires the involvement of more than one person who is flying contraception-free. It feels a little easy, not to mention predictable, to blame the woman for getting pregnant unintentionally when there is, presumably, a guy in the background who doesn't like to wear a condom, assumed she was on the pill and, hey, let's just get this show on the road already, yeah? Young men today have been spoiled in this regard, growing up in an era when all women of childbearing age in the US and the UK have legal access to contraception. These men also have easy access to porn in which they see only condom-free sex. To say that women should be strong and insist that their partner wear a condom if they themselves are not protected against pregnancy is sometimes easier to say in theory and harder in practice, especially if the young woman is inexperienced and feels that she should impress the guy.

Figures released earlier this year indicate a lax attitude towards contraception, especially among young people. According to Public Health England, sexually transmitted infections (STI) rose by 5% between 2011 and 2012, with people under 25 experiencing the highest rates. As Lisa Power from the Terrence Higgins Trust told the Huffington Post, using a phrase similar to the one in the Guttmacher Report: "Everyone knows about STIs but still think it won't happen to them."

When discussing US and UK contraceptive issues, those in the UK will always have it somewhat easier, thanks to relatively easy access to free contraception and advice. (Despite what some Republican politicians suggest, Planned Parenthood clinics are hardly ubiquitous in the US.) But what young people in both countries still need, it seems, is uncompromising education telling them that just because they've never been pregnant before doesn't mean they can't get pregnant; just because they're not a gay man in the 1970s doesn't mean they can't get an STI; and to teach young women to be more assertive with their partners. Because it really can happen to you.

While researching this story, I emailed Mary and asked after Holly, her younger sister and fellow Period Tracker devotee. Her baby's due this spring.