Hi. I've just had my letter back with results and trying to u seedy and how it's all done. This is my third pregnancy. I'm 33. Slim. Healthy ( as far as I know ). Don't drink or smoke. No illnesses in family to our knowledge. Both my previous pregnancies had substantially lower odds. ( one at aged 28 and one at 31) had 1 in 30000/1 in 15000 result for Downs. This one has come back as 1 in 4500. Says low risk still but I'm shocked at how much the odds have increased. Does anyone know what it's based on other than the fluid measured on the back of babies head ?