At least in the US, the declining fertility rate seems to be entirely caused by a combination of fewer people getting married and fewer people having children out of wedlock. Between 2007 and 2023, the percentage of reproductive-age women who were married fell from 42% to 37% and the births/1,000 unmarried women fell from 41.0 to 30.3, but the births/1,000 married women actually went up a bit from 102.3 to 107.4. See https://read.dukeupress.edu/demography/article/63/2/695/4092...