High risk for chromosomal abnormality?

Tookie
Hi all - has anyone else been told they are high risk for carrying Babies with Downs or other chromosomal abnormalities? My OB called yesterday to tell me my blood test results placed me in a high risk category but that there is often a high false positive with twins. I'm not freaking out yet- just curious how many of you also have been told this. I'm 17 weeks. Thanks!