Cigarettes are bad for many reasons. Not only are they packed with carbon monoxide, an odorless gas that reduces blood's ability to circulate oxygen throughout the body, they're also filled with addictive nicotine, sticky tars that coat the lungs, and they're precursors to emphysema and lung cancer [sources: Wise Geek; UPMC]. Over time, as people inhale smoke from a cigarette and tar builds up in the lungs, the number of functioning cilia in the lungs is reduced. This can be extremely harmful for a fetus. It can prevent the necessary amount of oxygen from reaching the baby, speed up an already rapid heartbeat, raise the risk of miscarriage, premature and still births, and future lung problems [source: WebMD]. Alternative nicotine sources aren't necessarily any better for a baby's health. So when it comes to smoking and pregnancy, the wisest and healthiest option is to simply kick the habit.