Why is it unsafe to smoke while pregnant?

Less oxygen to baby. The nicotine and other substances in cigarettes do a number on the blood vessels in the body, making them smaller and decreasing the blood flow. The blood flow to the uterus takes oxygen to your baby. By decreasing the oxygen to the baby, smoking can cause small babies, miscarriage or stillbirth.
Damages placenta. The simplest answer is it causes the tiny blood vessels in the placenta to shrink & over time will drop the babies weight at birth by a pound. The nicotine also affects babies brain as a stimulant, causing irritability of the nervous system. After delivery, the nicotine in babies system metabolizes very slowly, taking up to 2 days to drop by half.