Examples of errors:
(1) If the mother is a drinker the fetus drinks the same as its mother does.
(2) They say, "My father gave me a good spanking when I was young, and I believe in that, and I will do that same again."
The same in (1) is ambiguous, since it could represent either the same amount (of alcohol) or the same thing (i.e. alcohol). In contrast, either of the following would be acceptable:
If the mother drinks a bottle of whiskey a day, the fetus drinks the same (amount).
If the mother drinks whiskey, the fetus drinks the same (thing).
The problem in (2) is that do the same is a fixed expression and that is not possible here.