8 week sonogram

why do some mommies get 8 week ultrasound with the baby showing so much like u can see the head and lil arms and legs and then some u can't tell what it is just looks likes a big oval? i want a gummy bear shaped pic lol tomorrow i have a sonogram ill be 8 weeks and I'm praying ill see lil arms and legs lol does it matter if its abdominal or vaginal??