"I never dreamt of having kids," says Trump, after revealing her scars at the start of the film. "Now, I'm angry that, as a woman in my 40s, I have to make this decision. I never confessed to having second thoughts."
When Trump, 49, started making To Kid or Not To Kid, she hid behind the camera, asking other people about their decision to start a family. But she soon realised she needed to confront the question herself.
"I wasn't being very brave," she says. "I was afraid to talk to people about it, because I wasn't seeing my thoughts reflected back in society. I didn't know anyone in my close circle of family or friends who had made this decision."
High profile women, such as Oprah Winfrey, talk about their decision not to have kids, but it's more difficult for those not in the public eye. For years, Trump couched her decision to delay having a family with concerns about her career – she needed to be able to travel to Alaska and film for 10 days straight without contact to the world.
"I didn't want to be an absent mother," she says. "I was aware of what I'd put a child through." Her husband offered "to be the mum", but it wasn't enough.
From people asking at her wedding when she planned to start a family, to those who contemplate how difficult it must be for her to make friends without the school gates to help, Trump has confronted stigma for most of her adult life. On social media, she is often trolled with comments such as "What's your life worth if you don't have kids?", or: "You should be having kids, it's what God expects of you."
The belief that people should have children is systemic, says Trump, which makes it difficult for people to be open about their choices. In making the film, she unearthed government adverts encouraging people to have children.
"There's a Scandinavian one with a grandma actually undoing someone's bra," she says, as well as an Italian one celebrating Fertility Day. "And an Indian one saying: 'Be responsible, don't use a condom.' How can these governments have any involvement in our reproductive choices?"
"It's a myth," she continues, that more children are needed to tackle the problem of an aging population. "I live in New York, my mum's in Wales. It's not the same as it was 15 years ago when you might have your grandmother living with you. We have a global workforce now."
Nevertheless, women who don't have children are stigmatised and multiple people she approached refused to go on camera because they feared backlash in their social and professional lives.
"It's deemed to be hating children," she says, adding that she loves kids. "There are all these myths."
The tide is turning, says Trump, with younger generations feeling more comfortable having the discussion about whether or not they want children. Part of the reason is a growing awareness of the environmental impacts of having children. It comes as people choose to eat less meat, cycle rather than take transport, and use recyclable products, she adds.
"Six years ago, when I started making this film, that wasn't anywhere," says Trump. "There seems to be more of a passion for working this out. I feel it with younger people, which is really exciting to see, as someone who hasn't been able to talk about not having kids. It feels like the taboo is starting to break."
Trump is partnered with Population Matters, a charity that campaigns for people to have smaller families to tackle climate change, of which Sir David Attenborough is patron. It encourages people to only have two children or fewer to stop population growth.
"I was ambivalent about having children, so protecting the environment has been part of my decision," says Trump. "On average, if we have half a person less in family size, then we can reduce the population."
Trump wants more people to really interrogate their decision to have children. "The film is trying to get people to take the time," she says, adding that 45 per cent of pregnancies in the UK and US are unplanned. "Let's not bring unwanted children into the world. That's a big issue that's really at the heart of this film."
At last, it is a pregnancy scare that helps Trump decide to be "childfree". In the film, we watch as she and Granger come to the realisation they may have not been careful enough one night. After a tense conversation, she decides to get the morning-after pill – and he agrees to have a vasectomy.
"I don't call myself 'childless'," she says. "For women who can't have children, there's a grief process. I didn't have to go through that. I was able to celebrate the identity in the end. And I want other people to feel that way too, to own their decision."
But it was difficult telling her elderly mother, who always hoped for more grandchildren from Trump. She believes her daughter's decision was influenced by her teenage health problems – but Trump disputes this reading.
"The door was always open," she says. "I didn't want it enough."
While Trump's mum comes to terms with her daughter's decision, the filmmaker is grateful to her sister, whom she supported as a single mum, for giving her mum grandchildren and great-grandchildren.
"I feel for those who are only children," she says. "I have it easier than a lot of people."
Trump hopes To Kid or Not To Kid will encourage people to be more open and to replace the taboo over people who choose not to parent with celebration. "If you desperately want children, if it's one of your life's dreams, then I'm happy for you," she says. "In the same way I want you to be happy for me."
Since she made the film, countless people have approached her to say they didn't realise they had a choice not to have children. But the best moment was when someone congratulated her for being childfree.
"It made me tingle all over," she says. "I congratulate quite a lot of people now. It should be congratulated in the same way people celebrate in a huge way when you have a baby."
