

In the past, people had several children because most would die before adulthood. The 20th century population boom is because better sanitation and healthcare reduced child mortality but it takes at least one generation for women to adapt and have fewer children.
I do not disagree with you on the reasons why women have fewer children. I think there is also a significant cultural shift in the number of children women were supposed to have. In pre-industrual Europe, women were expected to be quickly married and then have lots of children with their husband. Women today can enjoy long careers and fulfilling lives without marriage or a family while such options were not available to women of the past.