It is a theory based on which side the baby implanted on (so placental location and NOT uterus) and one that can be theorized extremely early in the pregnancy. You would need an ultrasound of 6-8 weeks, and ask your provider where the placenta is located/implanted.
If the baby implanted on the left = girl!
97.5% of females implant on the left
If the baby implanted on the right = boy!
97.2% of males implant on the right side
Be careful, though! Depending on which style of u/s you had done, it can change the results. If you had an abdominal ultrasound, it is mirrored. So if your placenta is on the right, that actually means it's on the left (which would indicate GIRL). If your placenta is on the left, that means it's actually on the right (Which would indicate BOY). If you had a transvaginal u/s, everything is exactly how it is in your uterus, so left placenta = GIRL and right placenta = BOY.
Remember though that this is NOT fool proof and it's more for FUN! The ONLY sure proof way of finding out baby's sex is at birth ;)
**update: It's been confirmed that is IS indeed a boy! Senna Neil is due in March 2015 & we can't WAIT to meet him!!**
Edit/Update 03/23/2015: I recently gave birth to my son, Senna Neil, via repeat emergency cesarean. Not only am I still healing from surgery but I'm taking care of a newborn, a toddler, and an autistic child. Please be patient as it may take some time for me to reply to you. TIA!