Not manage to become pregnant, say? Bad idea (13)
I believe the answer is:
misconception
'bad idea' is the definition.
'not manage to become pregnant say?' is the wordplay.
I cannot really see how this works, but
'not' could be 'con' (I've seen this in other clues) and 'con' is found within the answer.
This explanation may well be incorrect...
(Other definitions for misconception that I've seen before include "Wrong view or opinion" , "Mistake" , "False view - economic pints (anag)" , "Delusion - wrong idea" .)