Many studies since then have corroborated evidence that women who smoke are about twice as likely to have a baby whose weight at birth is abnormally low. On average, the weight difference is about 200g (7oz) when compared with the babies of non-smokers.
A baby’s normal weight gain and development depend on a good oxygen supply. All nourishment and oxygen come via the baby’s mother through the placenta. For nine months everything she eats, drinks or smokes has some influence on the baby growing inside her.
The developing fetus does not need cigarette smoke. It contains more than 2,000 different chemicals and gases including nicotine, carbon monoxide, ammonia, hydrogen, carbon dioxide and hydrocyanic acid. They are absorbed from the lungs into the bloodstream and from there, when a woman is pregnant, they reach the developing baby.
Carbon monoxide, for example, enters the bloodstream where it latches on to the pigment in the red cells known as hemoglobin, whose main function is to carry oxygen to all the tissues of the baby. This is essential for their nutrition and survival.
Carbon monoxide is a persistent gas replacing the oxygen, thus reducing the oxygen-carrying capacity of the blood. In heavy cigarette smokers about 7 percent, on average, of their hemoglobin carries carbon monoxide instead of oxygen.
When a pregnant woman smokes, the carbon monoxide and nicotine from a cigarette cut down her baby’s oxygen supply and retards its growth. Babies of smokers are on the average “small for dates”, that is 200grams lighter at birth than babies of non-smokers.
This means that they are not just smaller but also underdeveloped, and liable to a greater risk of complications in the first weeks of life. Having a smaller baby does not necessarily mean that the birth will be easier.
It also has been shown that mothers who smoke have a greater chance of miscarriage in the late stages of pregnancy, or of having a stillbirth, than non-smokers. They also run a greater risk of having placental problems, bleeding during pregnancy, premature rupture of membranes or preterm delivery. And, obviously, the more the mother smokes the greater the risk.
The risk to the child doesn’t stop at birth. If a mother smokes during the first year of a baby’s life she puts its health at risk. The child is far more likely to be admitted to the hospital with a chest infection than the baby of a non-smoking mother.
Even after the baby has been weaned, smoking can affect it. Like anyone sharing space with an average smoker, the baby could effectively smoke the equivalent of one cigarette an hour by inhaling the smoke-laden air.
There are long-term effects, too. By the age of seven, children of smokers are likely to be slightly smaller than the children of non-smokers and to perform less well in intelligence tests.
Another fact which becomes important later on is that children of mothers who smoke are more likely to develop the smoking habit. Children do learn by example but at least the child who takes up smoking because of parental example has some choice. The unborn child has no choice.
If a woman stops smoking at any stage during her pregnancy it will benefit her baby, and virtually all ill-effects are eliminated if she stops before the fourth month of pregnancy.
Mothers-to-be who reduce their smoking to five cigarettes or less a day are more likely to have healthier babies. Better still is to give up.