One little thing I think has been missed here: the question is asking if you'd prevent your child from being TG.
That could mean anything. TV, cross-dresser, andro or anything else covered by this forum. Not everyone who's TG feels the need to go the HRT/SRS route. If it was possible to detect a TG baby that early, would they then be able to tell what kind of TG? Would they know how to treat the baby so it developed properly? How often do we get frustrated by politicians or other authorities who think they know how to fix a problem when they're actually clueless? How, therefore, would you know that by preventing this development in the baby that you wouldn't make it worse?
Assuming that it would mean giving them the right body from birth - I still wouldn't. Maybe I've just been lucky, but as confusing and painful as it's been, I've also learned so much. I've become less judgemental, more understanding and more angry at the bigots ruining it for what are, at the end of the day, just people trying to live their lives.
There was a period recently where I kept having this dream, where I was fully transitioned, married and we'd gone for IVF. The resulting baby was intersex, and I'd end up in an argument with the docs who wanted us to "choose a gender" for the baby. Among my argument was "how is it I have to jump through all these hoops for surgery, yet you're perfectly happy to do this on a child who's too young to choose for hirself?" Don't know why or where that even came from, but this thread just reminded me of it *shrug*