What's different for Katelin is she's so sure kids are off the table she's considering sterilisation. The procedure isn't something to be taken lightly: it involves blocking the fallopian tubes, which then prevents the woman's eggs from reaching sperm and, yeah, you know the rest. In the UK it can be carried out on the NHS under local anaesthetic, but, most importantly, it's permanent (or difficult to reverse, at the very least).

Katelin, from Philadelphia, is absolutely certain she doesn't want children. No way. Not a chance in hell. "I honestly don't like kids – they're germ-y and annoying and gross," the 19-year-old explains. Which is a fair point: humans under five are messy, unpredictable things. But, then again, most adults never quite grow out of that stage either.

"This might sound a bit graphic… but the only unusual thing is now that my sperm ducts are in two sections; I have four sensitive areas [rather than two]. I've had no other long-term issues, though, and I've been able to have sex afterwards. It all works perfectly normally, you know?"

Today, one in five British women will never have kids, up from one in ten in the 1970s. And despite the number of vasectomies falling nearly two-thirds in the last decade, men are also vocal about their child-free choice. Like 29-year-old Paul Pritchard, for example, whose vasectomy is actually filmed live during the documentary. "I've never thought of myself as a father," he explains. "Children have never factored into any of my long-term life plans."

Some might argue this is a particularly extreme option for someone so young, but Katelin is unwavering in her opinion. "I'm old enough to vote, play the lottery, drive and go to jail – why can't I decide I don't want kids?" she asks. It's a good argument, and she's not alone in feeling like this: there is a growing number of men and women, in the UK and US, who are permanently altering their fertility. This phenomenon, known as the "child free movement", is the subject of a new BBC Three documentary, Young and Sterile: My Choice, exploring why teenagers and 20-somethings are advocating childlessness by choice, despite not already having children of their own.

People who choose to be sterilised are doing so for a whole bunch of reasons, not just out of fear of becoming accidentally pregnant or a father. Genetics is a key factor. Katelin has mental health problems and a serious heart defect she doesn't want to pass on (though she also enjoys her "vagina exactly the way it is".) Equally, Pritchard has suffered from depression throughout his life and lives with Type 1 insulin-dependent diabetes: "It would be cruel of me to enforce a child to suffer the same," he says.

But finding a doctor who will actually agree to the procedure is where the difficulty starts. It took Pritchard 11 years before the NHS gave in to his requests. As a man in his twenties, he's considered old enough to be the father of unlimited children without checking with anyone first. But tell the NHS you genuinely don't want that, ever? It becomes a rejected, defeated world of long-term contraception – an assortment of tablets, injections and devices – to achieve the same ends.

Andie was sterilised last year and says it's the best decision they've ever made. "It's massively improved my mental health and [body] dysphoria. No one ever stops people from having children so why would you stop someone who doesn't want children?"

For 28-year-old Andie, who chooses not to identify as a specific gender (and uses the pronoun they), the choice stems from something stronger. "My mother was a really violent person and I was excommunicated from her at a young age," says Andie. "I'm scared of having kids and turning into her, because motherhood was quite a cruel thing for me. I wouldn't want my kids to go through the same experience."

"My first doctor, who has now retired, definitely fed me some bullshit, made-up statistics about how 90 percent of people who have a vasectomy regret their decision," explains Pritchard. "It's like, you're just telling me that because I'm 18 and you don't believe a word I'm saying."

"This idea that women have to settle down, get married, have children – it's quite a moral thing, isn't it? Life isn't just about reproduction."

The stigma around simply not wanting children is huge, but it seems to be worse for women. There's societal pressures, sure, but a woman lacking any maternal instinct? That's a step too far for some – it's weird, heartless and, well, outright selfish. For Andie, who was born a woman, other people felt as strongly about their decision to be sterilised as they did. "People judge you," they explains. "I've been back to hospital, unrelated to my procedure, and female doctors have lectured me on something I've already had done."

Andie believes this resistance is a symptom of defined gender roles. "This idea that women have to settle down, get married, have children – it's quite a moral thing, isn't it? Life isn't just about reproduction. For me it goes back to this patriarchal idea that gender is binary and, within that, you have to comply with set or 'normal' roles. I'm very much against that."