Blank "father" lines were generally required by the 1970s for unmarried women's children in most states, because states would use birth certificate paternal data to chase down unmarried fathers for child support. As a result, an unmarried father now has to give in-person consent for his name to be listed on the birth certificate; in some states, this requires a court proceeding, not just signing the birth certificate when the baby arrives.
There's a whole story worth writing about how American laws changed in the 1970s and early 1980s to allow for alternative child-naming practices. Most states used to require that an unmarried woman's child bear her last name and that a married woman's child bear the father's last name. I've got a case in my files from Georgia in the 1970s where a married couple had to sue the state registrar of vital statistics for the right to give their child a hyphenated (mother-father) last name. (They won.)
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There's a whole story worth writing about how American laws changed in the 1970s and early 1980s to allow for alternative child-naming practices. Most states used to require that an unmarried woman's child bear her last name and that a married woman's child bear the father's last name. I've got a case in my files from Georgia in the 1970s where a married couple had to sue the state registrar of vital statistics for the right to give their child a hyphenated (mother-father) last name. (They won.)