There are other factors too, such as many young married couples putting off thinking about having children. And some put it off too late.
When are women most fertile? In their late teens and early 20s I would guess? Yet some women, especially if they are getting into professional careers, don’t even think about having children until in their 30s. And fertility has already started declining for many women by age 30, from what I hear.
Their marriage prospects decline a good bit at 30 also.
Not surprisingly, women are most attractive to men when they are most fertile.
Abolish the welfare state. Replace it with parents getting a percentage of the taxes paid by each kid. Suddenly you provide an incentive to have lots of productive kids.