CREATE TABLE table_6449 ("Birth" TEXT, "Marriage" TEXT, "Became Duchess" TEXT, "Ceased to be Duchess" TEXT, "Death" TEXT, "Spouse" TEXT)

What is the Birth of the lady that has a Became Duchess on 18 august 1593 husband's accession?
SELECT "Birth" FROM table_6449 WHERE "Became Duchess" = '18 august 1593 husband''s accession'
SELECT "Birth" FROM "table_6449" WHERE "Became Duchess" = '18 august 1593 husband\'s accession'