CREATE TABLE table_name_52 (birth VARCHAR2, became_duchess VARCHAR2)

What is the Birth of the lady that has a Became Duchess on 18 august 1593 husband's accession?
SELECT birth FROM table_name_52 WHERE became_duchess = "18 august 1593 husband's accession"
SELECT "birth" FROM "table_name_52" WHERE "18 august 1593 husband's accession" = "became_duchess"