MATERNIT21 ACCURATE???

Is MaterniT21 accurate?? I had my results done and today i got the results, They said is a Boy!! But I had heard that some people were told it was a girl and ended up having a boy, My results came back and Its A BOY!!! but can this test detect a Y chromosome from my previous pregnancy Is Been 9yrs Ago and is Also a boy! But i had heard that Boys leaves ADN on the Moms Brain is it True???