Studies have estimated that between a quarter and a third of Australian women smoke during pregnancy.(107) Pregnant teenagers are also more likely to be smokers, reflecting coinciding patterns of high risk behaviour.(108,109)
Smoking affects the well-being of the foetus and the pregnant woman. According to the 1990 report of the US Surgeon General, 'Smoking is probably the most important modifiable cause of poor pregnancy outcome among women in the United States.'(2)
Nicotine, carbon monoxide and other toxic constituents of tobacco smoke cross the placenta readily, having a direct effect on the oxygen supply to the foetus, and the structure and function of the umbilical cord and placenta. A number of tobacco smoke constituents that cross the placenta are known carcinogens.(3) Nicotine has a direct effect on foetal heart rate and breathing movements.(56) Nicotine is also found in the breast milk of women who smoke.(56)
Spontaneous abortions and complications of pregnancy and labour occur more frequently in smokers.(97) Smokers have a higher risk of ectopic (tubal) pregnancy(110,111,112) and have a greater tendency to deliver preterm.(97) Women who smoke during pregnancy have a 25 to 50% higher rate of foetal and infant deaths compared with non-smokers.(1)
Exposure by the mother to workplace passive smoking and paternal smoking has also been associated with lower birthweight, a higher risk of perinatal mortality(113,114,115)and spontaneous abortion, particularly in the second trimester (mid three months) of pregnancy.(116)
Maternal smoking exerts a direct growth retarding effect on the foetus, resulting in a decrease in all dimensions including length and circumference of chest and head.(97) Infants of smokers weigh on average 200 grams less than the infants of non-smokers,(97) and smokers have double the risk of having a low birthweight baby.(1)
The reduction in dimension and birthweight observed among the newborn of smokers has been described in the United States as 'fetal tobacco syndrome', and is defined as follows:(1)
The mother smoked 5 or more cigarettes a day throughout the pregnancy.
The mother had no evidence of hypertension during pregnancy, specifically no preeclampsia and documentation of normal blood pressure at least once after the first trimester.
The newborn has symmetrical growth retardation at term, 37 weeks, defined as birthweight less than 2,500 grams, and a ponderal index (weight in grams divided by length) greater than 2.32. (Note: ponderal index is not a measure used in Australia).
There is no obvious cause of intrauterine growth retardation, that is, congenital malformation or infection.
Maternal smoking may predispose the child to respiratory illness. Parental smoking has been linked with decreased pulmonary function and asthma in children.(117) Smoking during pregnancy and in the infant's first year of life is considered one of the major risk factors for sudden infant death syndrome ('SIDS' or 'cot death').(118,119,120,121) Research has suggested that smoking by either parent during pregnancy is associated with a higher incidence of all childhood cancers combined, but especially acute lymphocytic leukaemia and lymphoma. There may be an association between paternal smoking and brain cancer.(122) However at this stage, these findings are not consistent and should be considered as tentative.(123) A recent meta-analysis suggests a small but statistically significant association between maternal cigarette smoking during the first trimester of pregnancy and increased risk of having a child with cleft lip/palate or cleft palate.(123a)
Other reported long term effects of maternal smoking on the infant include impairment of behavioural,(97,124) intellectual, and physical characteristics.(97) A recent study found a strong and significant positive association between cigarette smoking in mothers during pregnancy and attention deficit/hyperactivity disorder in their children.(97a) See also Chapter 4 for discussion of the health effects of passive smoking for infants and children. Readers requiring more information are referred in the first instance to references (56) and (125), both of which provide fully referenced, recent reviews of the medical evidence on the effects of passive smoking on pregnancy and infancy.