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_name_14 ( births__000s_ INTEGER, total_fertility_rate VARCHAR, natural_growth VARCHAR )

### Question ###
What is the average birth rate that has a total fertility rate of na, and a natural growth smaller than 2.5?

### Accurate SQL ###

SELECT AVG(births__000s_) FROM table_name_14 WHERE total_fertility_rate = "na" AND natural_growth < 2.5