Is it ever okay to hit a pregnant woman?

I was smacked in the back of the head today in class because I passed a test that nobody else did. So two girls thought it would be funny to smack me because I’m “not pregnant in the head”. (I wasn’t bragging or anything rude. I was helping another girl fix her math test when they hit me.) I understand they thought it was a joke and whatnot but I really didn’t appreciate it. I thought it was super disrespectful and I would NEVER hit someone who was pregnant on purpose no matter where it was at. I guess I’m just astounded that nursing students would find that funny. Anyway, they always make at least one comment everyday saying they can hit me somewhere because I’m not pregnant in that area. I’m just wondering if this is a common thing now? What do y’all think?