had another ultrasound today and my baby belly is still 2 weeks behind the rest of he.. i have had a…

had another ultrasound today and my baby belly is still 2 weeks behind the rest of he.. i have had a down syndrome test done and was positive with 1 soft marker which is no nasal bone. then got another test which i thought would tell me at 99 percent yes or no.. which came back negative for down syndrome. but no my doctor is saying she may be that 1 percent cause she is small.. what do u guys think about an ammio? am 26 weeks .. and very concerned