If she has given up her smoke then nothing bad will happen to fetus. if she continues, it might retard its growth.
Your Doc.can get an ultrasound
or give a physical check of abdomen to see size of fetus. generally there are some signs, girth of abdomen is measured to see if it corresponds with fetal height at 3 months and also every month and obstetrician comes to know whether growth is normal or not. Also fetus moves in stomach after few months, if no movement is discerned by mother, then report to doc.who will listen to heart sounds
of fetus by stethescope or a device and if anything wrong will ask for sonography
to assess the growth