Does smoking one or two cigarette per day really can harm my baby, pregnant 15 weeks?

Yes: Nicotine in any quantity will trigger the reduction in the size of the blood vessels of the placenta & eventually reduce the weight of the child. It is particularly toxic during the last trimester. If you can get down to 2/day you can probably stop by then. Breastfeeding smokers have decreased milk production & the kid has more infections & risk of crib death.