[Agent P]

Your parents might just want to make you wait, they might not even have a reason. Or it may just be way more exciting at 20 weeks.
Also, as Angelina said, it will be more accurate at 20 weeks than at 12.

Ultimately, it's up to you what you want to do. And I'm sure if you were persistent enough with asking why to wait, they'll answer eventually.
Or even ask your doctor what he thinks, as he should know. If he doesn't, I'd imagine your parents may just be picky or something.

Goodluck!