How to ask about ordinal place of an offspring?

Here is the question: I want to know what is the ordinal place of someone in her family. For example, I'd say:

I'm the second child of my parents.
and afterwards, I'd like to ask something along these lines:

What nth child are you? 

(So that I'm expecting something like first, second, third, etc. child as answer). Just wondering how the question should be asked.