No, there are at least three.
One is when people think they need to have a lot of kids for any of them to survive. The other is when human beings think their children will have it better than they do.
The third reason --- and I think this is one is seriously at play --- is that one needs children to secure old age. If mortality is high, there is a risk of not having any when you are old yourself. When poverty and risk are high, more children are necessary.
Socialism removes this need by assuring a person of support in his old age. But this is only a part of the story: the other is the overall wealth and a dramatic improvement in nutrition and medical care in the Western world.