My wife is pregnant by 18weeks and today usg showed low placenta by 22mm. do we need to worry about it and what is the normal range of low placenta...

No: I think anything less than 1.5 cm to the inner cervical os is considered a low-lying placenta--don't quote me on that. Either way, so early in the pregnancy, a low lying placenta is not very relevant; unless the patient is symptomatic (e.g. consistent vaginal bleeding). I would probably repeat a placenta focused study around 30/32 weeks. At that point, if the placenta is low lying; then worry.