How a 'fertility gap' is fuelling the rise of one-child familiesBBCNatalie Johnston was scrolling on Facebook a couple of years ago, when she came across a group called, "One And Done On The Fence". Seeing it, she felt a sense of relief."It was nice to hear someone giving it a name," she says. She and her husband have a five-year-old daughter called Joanie but they knew they probably wouldn't have a second child - not because they couldn't, but not out of choice, either: Natalie finds it hard to imagine having the time and money for one."You know you'd love that baby, everyone tells you, but there's a little teeny niggle where you think, 'what if I put my first in that position where she can't do the activity she wants to do because I've got to spread money out between two'?"She adds: "Is it okay to say you're only having one because they don't fit into modern ways of parenting?"Getty ImagesPeople aren't "turning their backs on parenthood", says the UN - some people simply can't afford it Modern parenting, for Natalie, 35, looks like family holidays with Joanie. It looks like weekday evenings hearing about her day at school and helping her with homework.