I am having a GIRL!!!!?

so i found out i was going to have a baby girl!! but i hear in ceratin cases they guess wrong!!?!?! what are my chances ??At the time of ultra sound my litto baby had her arms in between her legs so it was not very clear.. but no so probably a girl ??? ( i go to kaiser) hmm...anyways my secdon question... i am now on my 6th months and i STILL have not bought one piece of clothing for her... i looked around and it seems like the sizes run by how month old the baby is.. well at least the ones i saw.. there was preemies then 0-3 months, 3-6 months and so for.. preemies were little bit smaller then the 0-3 months.. but i wasnt sure what the differences were.. i am guessing preemies are right when they are born??? when they are the smallerest? how fast do they grow out of preemie size?? is it worth buying preemie size clothes?? or should i just buy 0-3 months for the start???