This is what I was telling you about Mary! Know the Sex of Your Baby at the FIRST Ultrasound! Or more specifically, the location of the placenta -- 97.2 percent of boys had their placenta on the right side of the uterus at that 6 week ultrasound, and 97.5 percent of girls had theirs on the left. Crazy, isn't it? According to their study, this method should be considered "highly reliable" in determining gender within the first few weeks of pregnancy.