Why 16 weeks??

I was just reading my pregnancy notes and it says...
- If any concerns BEFORE 16 weeks 
then call modwife for chat
- If any bleeding BEFORE 16 weeks then go A&E 
- If any concerns or bleeding AFTER 16 weeks then go to local maternity assessment unit.
I'm wondering why the difference for before and after this gestation? I mean being pregnant either before or after wouldn't that be maternity assessment automatically? Is 16 weeks a "special" mark stone or somthing?
 I'm almost 17 weeks so just wondering that's all.