Couldnt hear heart beat with doppler at 12 weeks!!!

I saw my midwife yesterday and i am 12 weeks and i asked her to listen to my baby for heart beat.... I couldnt hear it... Many ppl told me they coild hear theirs at 12 weeks.... Why couldnt i!? My midwife said she could hear the placenta moving and sometimes she could her a heart beat but thats because she knows what she was looking for . I am having my scan today, hopefully i can hear it or see it