Sense they're about to have a child (6)
I believe the answer is:
reason
'sense' is the definition.
(I've seen this before)
'about to have a child' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'to have' means one lot of letters go next to another.
'a child' becomes 'son' (male child).
'rea'+'son'='REASON'
'they're' acts as a link.
(Other definitions for reason that I've seen before include "issue" , "Cause; mind's power" , "Rational motive" , "Motive; argue" , "The right exercise of the mind" .)