How will smoking harm a growing baby?

Smaller babies. Sometimes babies are not only smaller but also sicker or less likely to handle other problems like infection. They also might be born early. Stop smoking now and ask your friends and family to stop also, before the baby is born and comes home.
Don't smoke period. Smoking in pregnancy is associated with several issues for a growing baby. It damages the placenta and decreases the flow of oxygenated blood to the baby. Babies do not grow as well and may have to be delivered prematurely as a result. It can also make the bag of water break too early. It also exposes your body and your fetus to toxins that aren't healthy for either of you.
Growth restriction. Tobacco smoke contains more tha 4000 chemicals, many of which affect crtical mechanisms for fetal health and growth.Three of the most critical chemicals are: 1. Nicotine: decreases blood flow to the placenta 2. Carbon monoxide: attaches to the fetla redblod cells and prevnet oxygen delivery to the fetus 3 cyanide: a cellular poison that prevents certain normal metabolic functions.
Don't smoke period. It harms fetal growth and oxygen delivery.
PPROM! The most devastating (and thankfully more uncommon) consequence of smoking during pregnancy is preterm premature rupture of membranes and preterm birth. This may lead to severe morbidity or mortality of the infant and scan families for ever. Having said that most pregnant smokers have normal outcomes and less common preeclampsia.