When a woman smokes during pregnancy her unborn child is an unwilling or passive smoker because it receives the effects of tobacco by-products through its mother's bloodstream. Studies have shown a number of effects on the foetus, and the growing infant, which endanger its life and health.

During Pregnancy

While a foetus is growing, it depends on its mother for all nourishment.However, the same mechanism which enables nourishment to be absorbed by the foetus can also allow some harmful substances to reach it. Carbon monoxide is a toxic gas in cigarette smoke which replaces oxygen in the smoker's (mother's) blood. Carbon monoxide also reaches the blood of the foetus. This reduces the amount of oxygen available for the survival and growth of all the tissues of the developing foetus.

A mother's smoking has an immediate effect on the foetus, accelerating the foetal heart rate and decreasing breathing movements.

Birth Weight and Foetal Growth

Babies born to women who smoke during pregnancy are, on average, 200 grams lighter than babies born to comparable women who do not smoke. Having an underweight baby does not make things easier for mother or baby at birth and puts the baby at risk. The more a woman smokes during pregnancy, the greater the reduction in birth weight. If a women gives up smoking by her fourth month of pregnancy, her risk of delivering a low birth-weight baby is similar to that of a non-smoker.

Babies of smoking mothers are small because they have made only slow growth over the nine months. Most of their organs weigh less than normal.

Some studies have shown that after birth the smoker's baby has a short period of accelerated growth, suggesting the removal of a toxic influence.

Foetal and Infant Illness and Death

Spontaneous abortion - Higher rates of spontaneous abortion (miscarriage) have been associated with smoking during pregnancy. In a study of 12,013 pregnancies in Ireland, a positive association was found between the number of cigarettes smoked and the rates of spontaneous abortion, independent of the effects of maternal age and number of previous live births.

Perinatal Death - Perinatal deaths include stillbirths, deaths during labor, and deaths in the first week after birth. One very large British survey showed a 28% increase in perinatal deaths when the mother smoked. The babies of women who gave up smoking early in pregnancy had a perinatal death rate similar to that of babies of non-smokers.

Complications of Pregnancy and Labour

Studies have consistently found a direct relationship between maternal smoking level and the incidence of placenta previa (development of the placenta in the lower part of the uterus so that it covers or adjoins the birth canal), abruptio placentae (premature detachment of the placenta), bleeding during pregnancy and premature rupture of membranes. The association is independent of socio-economic and racial background and many other factors. These complications carry with them a high risk of foetal and newborn loss, and are frequently cited as the cause of death among the offspring of women who smoke.

Childhood Illness and Death

Several studies have found that hospitalization rates for pneumonia and bronchitis were higher during the first year of life for infants of smoking mothers. Rates in children were higher if the smoking parents also had a cough and produced phlegm.

Maternal smoking habits have been determined in several studies of sudden infant death syndrome (SIDS). In all of these, an association has been found between maternal smoking during pregnancy and the incidence of sudden infant death. The infant's exposure to cigarette smoke (passive smoking) appears to increase the risk of SIDS. Whether exposure before or after birth is more important has not yet been determined.

Long-term Growth and Development

Possible long-term consequences of maternal smoking during pregnancy are also of concern. Follow up studies of infants born to smoking mothers have shown some continuing effect at the age of seven years. The large-scale British Perinatal Mortality Survey showed these children were one centimetre shorter in height, and 3-5 months behind in their reading, mathematics and general ability compared with children of non-smoking mothers. At the age of eleven there was still some difference, but this was less than at the age of seven. At age eleven other factors in the child's environment exerted greater effects than foetal exposure to smoking, but smoking was still an independent risk factor.

In Summary

A woman who smokes during pregnancy endangers the healthy development, birth and well-being of her baby. Studies have shown that smoking during pregnancy can lead to miscarriage, stillbirth illness and even death in early infancy. Other studies show that parental smoking may affect the health, physical growth and mental development of children up to the age of eleven.