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Smoking during pregnancy increases risk of preterm birth

Preterm birth, low birthweight, stillbirth and perinatal death are all associated with smoking during pregnancy.

The study, from Australia, included data from nearly a quarter of a million women who had two consecutive singleton deliveries (live and stillborn) during a ten-year period.

Relevant demographic data (eg parity, maternal age, birth interval, smoking status) and obstetric data (eg birth weight, gestation at delivery, gestational diabetes) were collected during each pregnancy. The smoking data enabled the authors to analyse the effects of continuation, reduction or cessation of smoking between the two pregnancies.

Women who smoked during both pregnancies had almost double the risk of preterm birth (8.4% compared with 4.1%, OR 1.89) and nearly three times the risk of low birthweight (8.8% compared with 2.8%, OR 2.92) in the second pregnancy compared with non-smokers. Women who stopped smoking before the second pregnancy, or who started smoking during the second pregnancy, had a higher incidence of adverse outcomes compared with non-smokers, but not as high as those who smoked during both pregnancies.

The adjusted odds ratio of both the risk of preterm birth and low birthweight showed a definite dose-related effect, with the risk increasing with the number of cigarettes smoked per day.

When stillbirth and perinatal deaths were analysed, gestational age, previous perinatal death and maternal smoking were all found to be significant independent variables.

Smoking is hazardous in pregnancy. It increases the risk of a small-for-dates baby, preterm birth and perinatal death. Its adverse effects are dose dependent, so it is worthwhile congratulating pregnant women who have managed to cut down.

The authors of the study state that ‘fundamentally, healthy pregnancies begin before conception.' So pre-pregnancy smoking cessation or reduction should be a priority.

Mohsin M, Jalaludin B. Influence of previous pregnancy outcomes and continued smoking on subsequent pregnancy outcomes: an exploratory study in Australia. BJOG 2008 doi: 10.1111/j.1471-0528.2008.01864


Dr Chris Barclay
GP, Sheffield

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