High hcg measuring ahead oddities in scan pic 2 heart beats. missed twins?

this is 5th baby measuring 2 weeks ahead on uterus when usually measure small with other babies hcg came back high on triple test other levels fine. picking up 2 strong sounding heartbeats in different places consistently on doppler. also looks like there could be another spine and head behind the baby in the scan and it doesn't look as if the leg belongs to this baby lol can they miss a twin on an ultrasound? going for a gender scan friday so will hopefully get my answer then but its driving me mad.a href="http://www.facebook.com/photo.php?pid=2197392&l=a550a8ef99&id=556658827" rel="nofollow"http://www.facebook.com/photo.php?pid=21…/a follow the head line and neck down to the spine it looks like somethings behind!