That has never been confirmed in the Bloshinskys' official narrative.
Jeanette *believed* she was pregnant with a girl, because of the way her body changed (in the same way as it did when she was pregnant with Ari, bot comparable to when she was pregnant with the twins). There are still many people today who blieve you can tell the sex of the unborn baby by the "way the mother carries" — none of them are scientific, classify them under "old wives tales".
On top of it, Jaron's sex could not be determined by the ultrasounds Jeanette got. Then her friends and family did the "
Pendulum Swing Test" (which has the reliability of a fortune cookie) and she was even more sure she was expecting a girl.
I don't think Jeanette
wanted a girl and
only a girl and that is why she was so adamant on Jazz's being transgender. There were more factors at play, in my opinion.
However, one thing that has always stood out to me, might be of importance as to why Jeanette
now (not from birth or in very early childhood) wants to keep Jazz dependent on her. When the twins were in their senior year of high school (not even in college yet), she was lamenting how she suffered from "empty nest syndrome". When Sander and Griffen finally moved out to attend college, they shared a room and didn't contact Jeanette as often as she'd liked them to. She was
not pleased with that. Both boys even dared to grow a beard without telling Jeanette and she was all up in arms about it when she discovered it on their next video call.
She has said plenty of times before Jazz's gradution was even near that she would miss her baby when Jazz would go off to college, because then all of her children would have left the nest. Greg and Jeanette even went condo shopping in one epidode as their plan was to move out of the house that would be too big for just the two of them. It might have been Greg's idea, though: he seemed content with smaller condos, but Jeanette wanted enough bedrooms in case her four kids came to visit them. They still live in their old house.
Having a "special child" was probably one of the more contributing factors for believing Jazz was transgender and the transition that followed than Jeanette's desire to have another girl.