Don't worry, you'll grow in time.
I know heaps of people who actually lost weight while preg and they ate healthy and everything (they were big girls though)
I am small like you although my pre-preg weight was 54kg. At 12wks i was 56kg, find out on Monday what weight i am now... But last app was 18wks and i was 58kg
I was told i would prob put on between 11-16kg. (i'm 164cm)