Gender

So I am going to be 13 weeks tomorrow and I found a place by my house that will tell you the gender early. I really want to do it because my doctor isn't going to tell me the sex until august or sept. The only thing I am concerned about is if I find out the gender early on Monday and let's say it's a girl but then the baby is still growing and forming body parts it turns out to be a boy later? I don't care if it's a girl or boy but I just don't want to be told one thing then another! Do you think I should wait for my doctor to tell me or should I take the chance of finding out early?