Information for Health Professionals

Maternal Smoking Whilst Pregnant

Every single cigarette a woman smokes affects her health, her pregnancy, and her baby’s health.

Smoking is the most important modifiable cause of poor pregnancy outcome among women.

A systemic absorption on toxins occurs whilst smoking when pregnant. 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. Nicotine has a direct effect on foetal heart rate and breathing movements. Nicotine is also found in the breast milk of women who smoke.

Spontaneous abortions and complications of pregnancy and labour occur more frequently in smokers. Smokers have a higher risk of ectopic (tubal) pregnancy and have a greater tendency to deliver pre-term. Women who smoke during pregnancy have a 25 to 50% higher rate of foetal and infant deaths compared with non-smokers.

Exposure by the mother to workplace passive smoking and paternal smoking has also been associated with lower birth-weight, a higher risk of perinatal mortality and spontaneous abortion, particularly in the second trimester of pregnancy.

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. Infants of smokers weigh on average 200 grams less than the infants of non-smokers, and smokers have double the risk of having a low birth-weight baby.

Maternal smoking predisposes the child to respiratory illness. Parental smoking has been linked with decreased pulmonary function and asthma in children.

The increased risk of reduced respiratory function, and increased risk of Asthma and Sudden Infant Death Syndrome (SIDS) is most marked in children of mothers who smoke heavily (more than 10 cigarettes per day).

Smoking behaviour characteristics amongst pregnant women and new mothers suggest that:

• women who smoke during pregnancy will continue to smoke postnatally
• of those who do not smoke during pregnancy many will start/resume smoking postnatally
• few mothers who smoke during pregnancy will then stop afterwards.

Evidence suggests that pregnant women need to quit smoking or reduce consumption during the first half of pregnancy to reduce the risks. Non-smoking expectant mothers should avoid exposure to cigarette smoke during their pregnancy in order to avoid many respiratory health problems in their child’s infancy.

SIDS

An increased risk of SIDS when babies are exposed to cigarette smoke has been found in over 30 case-control and cohort studies (Mitchell 1995, Golding 1997). This finding is consistent over time and place. Many studies have reported a dose-response relationship.

A recent case-control study in the UK (Blair et al 1996) carried out on families with infants born 1993-1995, since the change in sleeping position was promoted, found that the incidence of smoking during pregnancy was greater in mothers of 195 SIDS cases (63%) than in mothers of 780 controls (25%) (AOR 2.1 (1.24, 3.54)). If fathers were smokers then there was an independent additive increase in the risk of SIDS (AOR 2.4 (1.48, 4.22)).

If parents smoked in the house after birth, then there was an independent additive increased risk of SIDS (AOR 2.93 (1.56,5.48)). The population attributable risk from smoke of 61% is higher than the 33% reported for smoking prior to the reduction of prone sleeping (Mitchell 1995).

Reprinted with permission from the ‘Reducing the risk of Sudden Infant Death Syndrome (SIDS)’ booklet.