I think it will be an ever increasing trend.
1) A lot of people are having kids much later in life than they have historically
2) Both parents are moving into full time/professional jobs
3) Cost of daycare is omgwtfretarded expensive
Filling in the gaps the story is this...there's a lot of families that feel like like they need to have kids. Either because of societal norms, family, own pressure, whatever. And instead of doing that at 20-25 like people did over the last several decades they are waiting till they are more financially sound, get their traveling needs out of their system, ect. So now they are waiting till their 30's to crank out a kid. That already puts them behind the 8 ball of time.
So you get that bundle of joy in your 30's, put up with the sleepless nights, the $1500 a month daycare bill. That urge for the 2nd one gets slower and slower as your first one gets older and they start having meltdowns in the grocery store. You see neighbors down the road with shiny new cars or going on vacations. You look at the total look of bewilderment and defeat on the faces of parents at the mall or on vacation with multiple young kids. That decision to stick with one just seems all that much easier. The money, the lack of sleep, coordinating with work, the frustrations. Ect. Maybe one isn't so bad.
I know *a lot* of people in this exact situation.