Translation:My mother arrived pregnant in China, where she later gave birth to me.
My mother came to China pregnant, where she later gave birth to me. <-Marked wrong.
"alveni" (to arrive) is subtly different than "veni" (to come).
- "Mia patrino venis graveda al Ĉinio" = "my mother came to China pregnant" but may have given birth on the plane
- "Mia patrino alvenis graveda al Ĉinio" = "my mother arrived in China pregnant" (was still pregnant when she got there).
Maybe because "came" implies that you are in China yourself while you're talking about it, whereas "arrived" does not?
I wrote the exact same sentence as yours, down to every single word and got accepted.
Even though there is a difference between "veni" and "alveni", in this particular case, "came" is fine.
I've known people who did that to get their children dual citizenship...