trisomy 13, 18, and 21 question

Can anyone help me understand what implications testing positive for trisomy 13, 18, and 21 are? I know 21 is Down's syndrome, but I'm confused about 13 and 18. Online it seems like there is a million different things. Has anyone had experience with these? I'm getting tested in a few days and want to know a bit more (of course I will ask my doctor for more information as well 🙂)