It is supposed to be 99.9% accurate.  They take a blood sample from the mom and test for Y chromosomes because the baby’s blood crosses the placenta and mixes with the mother’s blood.  This means you have your baby’s DNA in your body.  IF it is a boy, they will find Y chromosomes (obviously cannot belong to the mom since girls are XX).  If it is a girl they will not find Y chromosomes.  We heard about it through the CBS Morning Show.