Box gave birth to Trixie, whose father is former Ironman Grant Kenny, in 2013.
The pair split after a brief romance in 2012 but are reportedly on good terms, with Kenny regularly visiting his daughter in Melbourne.
Box added that she was "blessed to be having a "much-wanted sibling" for her daughter.
“I know the value of this miracle because I have seen first hand the heartbreak, tears and sacrifices so many women and couples suffer on their IVF journeys and my heart goes out to those who are facing that pain and struggle,” Box wrote in her emotional post.
“It is not lost on me that I am one of the lucky ones.”