Just ask you GP for a referral for an ultrasound. I doubt they will refuse. Please be aware that ultrasounds are not perfect when it comes to calculating size but it should be able to confirm bubs is healthy. That should ease your mind.

BTW, I don't smoke and my DS was 4.3kg born. From every appointment from 18 weeks, the standard comment was "geez, your having a big one". This was not what a first time mum wanted or needed to hear. With this one (due in 2 weeks), I am a lot smaller and some comments have been "geez, this will be a small baby". Smoking may affect the growth of bubs but if you have done everything else to ensure a healthy bubs, then you have done your very best. Please try to enjoy the rest of your pregnancy.

It is obvious you love this baby and will be a great mum. Good luck.