Starting a baby is the idea (10)
I believe the answer is:
conception
I believe this is a double definition.
'starting a baby' is the first definition.
(as in conceiving a baby)
'the idea' is the second definition.
(conception is a kind of idea)
'is' is the link.
(Other definitions for conception that I've seen before include "See 5" , "Idea (maybe leading to baby)" , "Plan - birth" , "how to get pregnant?" .)