Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: CREATE TABLE table_6449 ( "Birth" text, "Marriage" text, "Became Duchess" text, "Ceased to be Duchess" text, "Death" text, "Spouse" text )

### Question ###
What is the Birth of the lady that has a Became Duchess on 18 august 1593 husband's accession?

### Accurate SQL ###

SELECT "Birth" FROM table_6449 WHERE "Became Duchess" = '18 august 1593 husband''s accession'