The earliest (depending on the person doing it and the machine they are using) my best friend who had twins found out at 16 weeks on the dot, i found out with my first at 21 weeks and with this one i was too excited lol
and found out at 16weeks 5 days and said i was having a girl but the 2nd time i had another ultrasound at 19 weeks they told me they weren't sure of the sex lol because there were "shadows" but didn't want to say for sure (for liability reasons), but then at 23 weeks they told me i was having a girl for sure
so i think it really depends
I'm sending you lots of sticky dust and i hope everything goes well for this pregnancy for you