At the other end of the scale, heavy smokers tended to have the fussiest and most difficult children, with high scores for poor mood and poor adaptability, likely precursors of antisocial behavior, Kate Pickett, Ph.D., of the University of York here, and colleagues reported in the Journal of Epidemiology and Community Health.
Quitting smoking during pregnancy appears to reflect the mother's personality and a deliberate effort to protect the baby rather than a long-term intention to quit altogether, inasmuch as postpartum relapse rates are extremely high, the investigators said.
Women who quit smoking during pregnancy have generally been grouped either with non-smokers or with continuing smokers in studies of prenatal smoking. However, the independent contribution of quitting to behavioral pathways has not been examined, the researchers said.
Although smoking during pregnancy has consistently been associated with a risk of behavior problems in the child, there is a debate about whether the association reflects teratological effects or is a marker for difficult maternal characteristics. The researchers suggested that it could be both.
To test the competing hypotheses, the researchers undertook a study of more than 18,000 British babies born from 2000 through 2002, who were participants in the Millennium Cohort Study.
Their mothers were classified as either non-smokers during pregnancy, quitters, light smokers, or those who smoked 10 or more cigarettes a day (heavy smokers).
The infants' temperaments were assessed when they were nine months old, using the validated Carey Infant Temperament Scale. The test was designed to pick up positive mood, receptivity to new things, and regular sleep and eating patterns (regularity).
More than a third (35.7%) of the mothers reported smoking at some time during pregnancy, although almost half of these quit smoking. Only a small proportion of the sample (9.5%) smoked heavily throughout pregnancy.
Women who smoked through pregnancy had significantly smaller infants and shorter gestations. The women who quit smoking had infants with birth weights and gestations comparable to non-smokers.
With never-smokers as the reference, quitters had infants with the highest scores (57.3) of easy temperament while heavy smokers had infants with the lowest scores (55.7) (F=28.51, P<0.001).
However, there was no significant difference in the pair-wise contrast between non-smokers and heavy smokers (F=2.20, P=0.018), with infants whose mothers quit smoking having a more positive mood.
Pregnancy smoking also predicted difficult temperament in the child and a tendency toward an increased risk of low positive mood (OR=1.17, P=0.09).
In contrast, quitting exerted a protective effect with decreased risk of distress to novelty (OR=0.79, P <0.01) and irregularity (OR=0.89, P=0.02) in these infants.
Pathways from pregnancy smoking to offspring behavior are complex and multi-determined, the researchers said. Pregnant smokers not only pass on toxic chemicals, but also traits and types of behavior associated with their continuing to smoke during pregnancy, the researchers said.
The capacity to suspend smoking is likely to be one manifestation of maternal personality characteristics, such as the capacity to adapt to environmental circumstances and the ability to plan and delay gratification.
Consistent with this, compared with pregnancy smokers, women who quit smoking in pregnancy had a better general capacity to function, including more sustained relationships, more skill in use of community resources, less stressful life circumstances, and were less likely to have a history of antisocial behavior.
These differences in functioning may reflect stable differences in personality that are linked to a protective behavioral style in the infant both through heritable processes and association with good-quality parenting and the family environment, the investigators said.
Thus, the evidence suggests that both maternal characteristics that distinguish persistent smoking from quitting and teratological effects are potentially at work, the researchers said.
In particular, the association of quitting with easy temperament in the child suggests that both quitting and easy temperament may reflect heritable, stable aspects of personality encompassing flexibility, regularity, and positive mood in the mother.
As women who quit smoking in pregnancy are exposing their unborn infants to some degree, and as many will subsequently relapse and expose their infants to environmental tobacco smoke, these findings suggest that maternal characteristics can potentially override the teratological effects of low-grade or intermittent smoke exposure, the investigators said.
So the capacity to suspend smoking on behalf of fetal well-being appears to represent not only the absence of problem smoking behavior but also adaptive maternal characteristics such as self-restraint and the ability to shift behavior with the changing environment.
Study limitations included the sole reliance on maternal report of infant temperament, while assessment of maternal smoking was also brief, self-reported, retrospective, and did not capture the fluctuation in smoking common in pregnancy.
Fundamentally, these findings underscore that pregnancy smoking, or quitting, is not an individual but a complex maternal behavior pattern. Enhancing the understanding of this complex phenomenon will inform research on prevention and effects on the offspring, the investigators concluded.
|This research was supported by a postgraduate Advanced Studentship in Health Services Research from the Medical Research Council, UK. Dr. Pickett and others were supported by a grant from the U.S. National Institute for Drug Abuse.
No financial conflicts of interest were reported.
Zalman S. Agus, MD Emeritus Professor
University of Pennsylvania School of Medicine
Journal of Epidemiology and Community HealthSource Reference: Pickett KE, et al "Meaningful differences in maternal smoking behavior during pregnancy: Implications for infant behavioral vulnerability" J Epidemiol Community Health 2008; 62: 318 - 324.