Smoking in pregnancy can harm a growing baby - before birth and in the future. Smoking raises the levels of carbon monoxide in the mother’s bloodstream, restricting the amount of oxygen available to the fetus. Nicotine also constricts the blood vessels to the placenta, further reducing the baby’s oxygen supply. As a result, the baby grows less well and will likely have a lower birth weight - increasing the risk of medical problems following birth.
The brain development of a growing baby is also believed to be inhibited if the mother smokes. This may last into childhood and beyond. Recently, researchers have pinpointed the second and third trimesters as the “danger period” for fetuses exposed to cigarette smoke. This is good news as it gives a woman who has problems quitting, a longer time to stop after learning she is pregnant. There is also evidence that second-hand smoke can affect an unborn baby's health, although not to the same degree as if the mother smokes.