Time was when people trying for a family would weigh up whether they were with the right partner, if they had the financial means to bring a child into the world, and how ready they were to commit to 18 years of hard parenting.
But another consideration has just leaped to the top of that list: what will be the environmental impact of us having children?
This week, the Duke of Sussex announced that he and the Duchess will have "two children, maximum", for the sake of the planet and its ever-dwindling natural resources. In a discussion with environmentalist Dr Jane Goodall in the new "Forces of Change" issue of Vogue - which his wife guest-edited - the Duke revealed that becoming a father for the first time had brought home the "terrifying" effects of climate change, and that "what we need to remind everybody is, these are things that are happening now".
When Dr Goodall said there will be future conflicts over diminishing supplies of fresh land and water, he replied: "We are the frog in the water and it's already being brought to the boil."
Environmental campaigners - who have long maintained that having children is the single most destructive thing a person can do to the environment - were thrilled when the Duke brought the subject to the fore. Alistair Currie, head of campaigns at Population Matters, which counts Sir David Attenborough and Chris Packham as patrons, says: "It's fantastic that people with their profile are conveying the message that the root to environmental sustainability has to include smaller families."
The population is expected to rise to 10.9 billion by the end of the century, a figure environmentalists believe is unsustainable. A recent report from the Lancet Commission found it is "increasingly unlikely" that food systems will not cope once the population rises above 10 billion. Last year, the UK's population saw its largest annual increase in nearly 70 years.
"The best chance the world has is for people to have [a maximum of] two children," says Currie. Researchers at Lund University in Sweden suggested that having one fewer child per family can save an average of 58.6 tons of CO2-equivalent emissions per year, making bringing a child into the world far more environmentally destructive than driving, eating meat and air travel.
With growing awareness of the climate crisis, more people are opting to have fewer children - or none at all. BirthStrike, a pressure group for those choosing to have no children, has amassed 550 members since it launched in May, while a recent post on the "Population Matters" Faebook page about millennials not having children was liked by some four million readers.
But when pitting the ecological challenges facing the planet against a human desire to a family life, what wins the day? Here, three parents with impeccable eco credentials, reveal how it's not so easy being green.
'Our daughter tells other children to leave flowers alone for the sake of the bees'
Gregory and Katherine Hamilton, from North Somerset, with their only child, daughter Breanne, four
Gregory Hamilton is proud of the young royals for starting a discussion about family size. He and his wife, Katherine, 35, who live in north Somerset, chose to only have one child in order to limit their impact on the environment.
"Prince Harry's setting a good example on an important issue," says Hamilton. "When Prince William said he was having a third child, I thought, that's not good, because it says it's OK to have three children. If everybody did that, we'd end up without a planet quite quickly."
If they hadn't been so ecologically minded a couple, Gregory and Katherine would have loved to have had two children - they each have a sister and cherish having had someone to grow up with. "When I see Breanne's friends playing with their brothers and sisters, I think, have we stolen something from our child?" says Hamilton. "But at the end of the day, if we hadn't made that choice, we would be stealing from every other child that's born."
He strongly believes that "having fewer children is the single largest thing that any family can do" to help safeguard the environment for future generations.
The Hamiltons are the sort of family who turn waste orange juice cartons (which they don't buy very regularly) into plastic bricks; take compostable coffee cups with them when they're out; and rarely use disposable nappies.
As such, they hope that when Breanne, grows up she will have a positive impact on the planet. The four-year-old has already chooses to be vegetarian and drinks from a steel water bottle, rather than a plastic sippy cup.
"You can see the internal battle she has every time she sees a flower," says Hamilton. "She's a kid and just wants to pick the flower and own it - but she understands they're important to the bees. And she now tells other children to leave them for the bees."
Breanne's clothes come from Frugi, a sustainable company that pays workers fairly, and most of her toys are wooden or recycled. When she was younger, the Hamiltons bought her reusable bamboo fibre nappies. Although they require a lot of electricity and water for regular washes, he calculated that it was still better for the environment than disposable ones.
"That was the least possible fun you can have," he says. "We bought 50 nappies [which are sold on to another family afterwards] and for a very long time our washing line was just them. We were very happy when she got out of nappies at a young age."
The lifestyle choices might be small, but when added up across a population and lifetime, they start to have an impact. Hamilton, who is now a vegan save for the occasional egg, became a vegetarian at 13 after a friend questioned whether one person doing it could make a real environmental difference. He thought: "If everyone thinks like that, then nothing will change."
Hamilton maintains this steadfast belief, which aids him when his lifestyle feels cumbersome. "We do things that are a real pain in the bum, but if every family lived this way, it would solve a lot of problems."
Last year, he and Katherine decided to avoid air travel, because if everybody flew all the time, "the planet would be gone in 10 years". This year, they holidayed in Wales. "I'm looking forward to someone making an electric plane and I can go on holiday again," he laughs.
'I haven't bought anything new for my third baby'
Emma Ross, 33, an eco-blogger from north London and mother of two - with another on the way
For 33-year-old Emma Ross - who is known as Mamalina to her 42,000 Instagram followers - having an environmentally friendly lifestyle is enough to offset her desire for a large family. Known for championing cloth nappies, recycled toys and homemade oat milk, Ross is 39 weeks' pregnant with her third child.
"I get asked a lot: 'How can you call yourself an environmentalist if you're having children?'" says Ross. "It's a responsibility I take really seriously. I want to raise Earth-conscious people who will be the next generation of environmentalists. People associate children with a lot of waste and resource use, but it doesn't have to be."
Ross has three siblings herself and has always wanted a large family. She discussed the concerns of having a third child with her husband, a lawyer, and together they decided that adding another to their brood wouldn't be too damaging. For one, they plan to reuse nappies, clothing and toys from their first two children.
"I haven't bought anything new for this baby," she says. "New babies are a bit like weddings: everyone gets overexcited and rushes out to buy new things. But all that babies need is milk, to be kept warm and allowed to sleep."
Ross's children - Jack, five, and Sonny, three - will have second-hand uniforms, food from the back garden and toys made from natural materials.
"We've opted not to go to play groups so we can spend our time doing educational things," she says, adding that they go to the supermarket together, where she teaches them to buy things that aren't wrapped in plastic. She was recently criticised for sharing a picture of her child with a plastic dummy.
In response, she says: "Everyone has plastic in their lives. I'm not saying we should ditch all plastic; it's about being more conscious and why you're using more materials."
In the future, she hopes to get an electric car and introduce her children to a vegan diet. For their next holiday, rather than flying (they offset their last trip, to Australia, through the Woodland Trust), they will be going to Scotland or Cornwall.
'I'm not having a child so I can fight for change'
Blythe Pepino, 33, founder of eco pressure group BirthStrike and mother of none
A growing number of ecologically minded people think having even one child is too many. The "birth-striker" movement has earnt prominent followers, notably Alexandria Ocasio-Cortez, rising star of the US Democrats, who has asked if it is "still OK" to have children.
London-based Blythe Pepino, 33 and the founder of BirthStrike, decided last year that the environmental crisis is so pressing that she cannot, in all conscience, bring a child into the world. "One of the reasons I'm not having a child is because I want to focus on creating the systematic change," she says. "We've gone past the point of no return."
The decision has been difficult for her partner, Joshua Hallam, 28, and her mother, both of whom were excited at the prospect of her starting a family. Hallam admits he had loved the idea of having a child together - but is supportive of her decision.
Pepino's mother was "very upset", and is still grieving for the grandchild she may never meet. "Then again, she knows I'm fighting for my niece, who's nine," says Pepino. "A lot of my friends are starting to have children and it's wonderful - but I'm seriously worried about their safety and security in the future."
Formerly lead vocalist in Vaults, the electronica band behind the 2016 John Lewis Christmas advert, Pepino now devotes the majority of her time to climate activism. She hopes that through organisations like BirthStrike and Extinction Rebellion, of which she is an active member, we will start to see real change to the way we eat, shop and travel.
She doesn't give up hope that she might one day become a parent, either by having a baby or adopting. "If things improve, we still hold out for the possibility of having a child," she says. "But we're both so rational that we're not going to do it if we're going to be fighting for food in a couple of decades."
The Daily Telegraph