In the past children improved well-being of the average family, because more labour for their fields was almost always good, and children started working when being 5-6 years old. After industrialization children become detrimental, because they can't really help the family, while their upbringing and education became much more relatively expensive.

modern women have relative financial, social, and medical independence compared to what they had in the past and quite reasonably don't want to go through pregnancy and raising children as early in their lives or as often as women in the past did

GDR had higher birthrates than FRG, and socialist countries in general had significantly higher birth rates than the same countries after 1991.