It's a boy! Next one a girl (5)
I believe the answer is:
sonia
'girl' is the definition.
(woman's name)
'it's a boy next one a' is the wordplay.
'it's a boy' becomes 'son' (synonyms).
'next' is a charade indicator (letters next to each other) (I've seen this in other clues).
'one' becomes 'i' (Roman numeral).
'son'+'i'+'a'='SONIA'
(Other definitions for sonia that I've seen before include "Noa is really a girl" , "...O'Sullivan, great Irish athlete" .)