The massive change is the transformation of women's role in society and education.

Essentially for pregnancies at 25 years old or later there are no number changes. Women of that age have as many kids as they ever had.

But, pregnancies for very young women and teenagers has virtually disappeared. As women start having children much later, they simply have much less.

As a millennial in southern Europe, the average age of our mothers in early 20s.

35 years later, the average age of first time mothers in my group is 33+.

The article you're commenting points it out as well:

> But demographers have long shown that what really counts is girls’ education. Schooling means that girls gain more autonomy and a greater say in life’s decisions.