Has anyone had an undersized fetus?

If your fetus was undersized like mine (mine is undersized by 10 days and i am 20 weeks) what was the reason for it and what was the outcome? my doctor does not know why its undersized. no chromosomal abnormalities showing on u/s or quad screen. maybe just small? i also was smoking at first a lot the first couple months b4 i knew and then i cut down to 3 a day and then quit but still dont know if that is causing this. also i have been having slow weight gain that my doctor was concerned about a few weeks ago about 5 pounds in total and i am 20 weeks