Associations Between Smoking, Poor Pregnancy Outcomes Are Cumulative
Compared with their counterparts who have never smoked, women who smoke during one pregnancy and continue to do so in the next have roughly two times the odds of having a preterm second birth, three times the odds of delivering an infant with a low birth weight and nearly 50% higher odds of having a pregnancy that ends in perinatal death, according to a retrospective epidemiologic study conducted in Australia.1 Women who stop smoking after the first pregnancy have smaller elevations of risk. The likelihood of having a preterm birth or a low-birth-weight infant are also positively associated with the number of cigarettes smoked daily.
Using data from a surveillance system that captures most births in New South Wales, investigators identified women who had two consecutive singleton births during 1994–2004. Births were categorized as preterm if they occurred before 37 weeks of gestation, and infants were categorized as having a low birth weight if they weighed less than 2,500 g at delivery. Perinatal deaths were defined as the composite of stillbirths (deaths of fetuses having a gestational age of at least 20 weeks or a weight of at least 400 g) and early neonatal deaths (deaths within the first 28 days of life). Women were classified as never-smokers (if they did not smoke during either pregnancy), moderate smokers (if they smoked 1–9 cigarettes daily) or heavy smokers (if they smoked 10 or more cigarettes daily) for each pregnancy.
The 244,840 women included in analyses were, on average, about 28 years old at the time of the first delivery and 30 years old at the time of the second delivery. The large majority (87% for each pregnancy) began receiving prenatal care before 20 weeks of gestation. About 4% developed gestational diabetes. The first and second births were closely spaced (between 12 and less than 24 months apart) in roughly a third of cases, moderately spaced (between 24 and less than 36 months apart) in another third and distantly spaced (36 or more months apart) in the remaining third; in only 2% of cases were the births very closely spaced (less than 12 months apart). Some 19% of women smoked during the first pregnancy, and 18% did so during the second; of the former group, almost three-fourths smoked during the second pregnancy.
Overall, 5% of the women had a second pregnancy that ended in a preterm birth. In multivariate analyses, compared with never-smokers, women who smoked during both pregnancies had almost two times the odds of having a preterm birth (odds ratio, 1.9). The odds were also elevated among women who smoked only during one pregnancy (1.4 for each). The likelihood of a preterm second birth varied with the frequency of smoking: The odds ratios ranged from 1.2 among women who did not smoke during the first pregnancy but smoked moderately during the second to 2.0 among those who smoked heavily during both.
A number of additional maternal and pregnancy-related factors were also positively associated with the risk of a preterm second birth. Women’s odds were most sharply elevated if they had had a preterm, rather than full-term, first birth (odds ratio, 6.2) and if their births were very closely, rather than moderately, spaced (3.6).
Some 4% of women delivered a low-birth-weight infant after the second pregnancy. Continuous smokers had nearly three times the odds of this outcome relative to never-smokers (odds ratio, 2.9). Women who smoked only during one pregnancy also had elevated odds of this outcome (1.7–2.1). Once again, the increase in risk was linked to the frequency of smoking: Odds ratios ranged from 1.6 among women who smoked moderately during the first pregnancy and not at all during the second to 3.1 among women who smoked heavily during both.
Several other maternal and pregnancy-related factors also were positively associated with the risk that the infant was low-birth-weight. Most notably, a preterm second birth was associated with a dramatically elevated risk of delivering a low-birth-weight infant (odds ratio, 68.9). In addition, compared with their counterparts whose first infant had a normal weight, those who had had a low-birth-weight baby had nearly five times the odds of experiencing the same outcome a second time (4.7).
For nearly 1% of women, the second pregnancy ended in perinatal death. In a multivariate analysis that included gestational age, the only determinant of this outcome was gestational age itself: Compared with their peers delivering at 37 weeks or later, women delivering at 32–36 weeks had sharply elevated odds (odds ratio, 17.6), and women delivering earlier than that had astronomically elevated odds (455.8). However, when gestational age was excluded, women who smoked during both pregnancies had an elevated risk of this outcome (1.5), as did those who smoked during one or the other (1.4). The odds were also sharply higher if the two births were very closely spaced, as opposed to moderately spaced (4.6), and if the first pregnancy had ended in stillbirth (3.6) or early neonatal death (5.9), as opposed to any other outcome.
Smoking during the second pregnancy accounted for 15% of the risk of preterm birth, 26% of that of low birth weight and 10% of that of perinatal death. Adverse outcomes of the previous pregnancy explained much of the remaining risk of preterm birth and low birth weight.
Factors predating conception exert a large influence on the health of a pregnancy, the investigators contend; therefore, early identification of women at elevated risk for poor pregnancy outcomes and appropriate intervention hold promise as strategies for ensuring healthy pregnancies. The researchers note that smoking is a modifiable risk factor, and that both smoking cessation and reduced frequency of smoking were associated with comparatively better pregnancy outcomes. “Strategies to reduce the prevalence of [smoking during pregnancy] may include intense intervention for women who have had smoking-related adverse outcomes in a previous pregnancy, but primary prevention is probably more important,” they conclude.
1. Mohsin M and 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.x.