David Ricardo wrote about this in the late 1700s.

His thesis was that until people have achieved the level of financial security their parents had before having them, people do not marry or have children.

This level is obviously different for people of different social origins.

Rural Europe was still in the Middle Ages then, but I think Ricardo's thesis still fits the observable facts today better than conventional explanations pushed byvregime mouthpieces - like level of childcare interventions, or the percentage of women photographed with framed pieces of paper.