IS there a chance the doctors could be wrong?

I went with my sister today to the doctors to get an exam done and they did an ultrasound! and said they couldnt find a heartbeat is there anyway that they could be wrong, she is only 6wks pregnant i dont know when they should be able to pick up a heartbeat they did an internal probe ultra sound??we would like to hear from people and get advice before going ahead with a dnc