Could Smoking Be The Cause For Delayed Foetus Growth?

I am currently 35 weeks pregnant and have been smoking cigarettes throughout my whole pregnancy, around a half a pack a day. I had an ultrasound at 34 weeks 5 days and they said his head and length are measuring right on but his waist is skinny, measuring 2 weeks behinds. The ultrasound machine estimated his weight at 4 lbs 9 oz. which they said is the 18th percentile. If I quit smoking now will it help my baby plump up better or is the damage already done? I haven t smoked since the ultrasound 2 days ago.