Originally Posted by Tyrolean

Depends on the nationality of the parents and the plane!
Where the plane is registered that law will apply (same on ships).
So if it is a US plane it will get the US nationality.
The country under the plane does not matter.

With most nationalities the child will get the nationality of the parents.

Example:
German parents on board an American flight:
Child will get both German and American nationalities.

But:
If you are over the the 34st week of pregnancy you are not allowed to fly.
You will probably denied boarding!