No absolutes sure, but there is a clear pattern…

You mean the World where demography is crashing down? You are a clown

They adapt by having more children per couples, not by having more single mother families. At least that was the case after WWII and WWI, but nowadays couples cannot afford more than 2 or 3 children.

And single mother in iceland is totally different, they were a couple with children before the husband died.

Thats not true at all, its not like farm husbandry, human do not just bred like animals. Only a fraction of women would be willing to have kids without a long term partner.