Mothers who smoked during pregnancy had an increased risk of giving birth to a child with a hyperkinetic disorder

Mothers who smoked during pregnancy had an increased risk of giving birth to a child with a hyperkinetic disorder

ARTICLE IN PRESS OBSTE TRICS Mothers who smoked during pregnancy had an increased risk of giving birth to a child with a hyperkinetic disorder Linnet...

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ARTICLE IN PRESS OBSTE TRICS

Mothers who smoked during pregnancy had an increased risk of giving birth to a child with a hyperkinetic disorder Linnet KM, Wisborg K, Obel C, Secher NJ, Thomsen PH, Agerbo E, HenriksenTB. Smoking during pregnancy and the risk for hyperkinetic disorder in offspring. Pediatrics 2005; 116: 462^7.

OBJECTIVE To examine the association between hyperkinetic disorder and maternal smoking during pregnancy. DESIGN A nested case-control study. SETTING Population based in Denmark.

SUBJECTS A total of 3935 children born between 1991 and 1994. Cases were 170 children with a diagnosis of a hyperkinetic disorder recorded in the Danish Psychiatric Central Register and were classi¢ed by their ¢rst diagnosis as; disturbance of activity and attention (n=102), hyperkinetic conduct disorder (n=33), other hyperkinetic disorders (n=2) and hyperkinetic disorder unspeci¢ed (n=33). Controls were 3765 children selected from a 5%, randomly chosen, sample of the study population and were matched with cases for gender and date of birth. Excluded were children with childhood autism, Asperger’s syndrome, other pervasive developmental disorders or mental retardation. The history of smoking during pregnancy was recorded, and when maternal smoking information was not available the child was excluded. METHOD The relative risk of women who smoked during their pregnancy giving birth to hyperkinetic children was calculated. The variables of parent and sibling history of psychiatric hospitalization or outpatient treatment were also considered when assessing the association between in utero smoking and hyperkinetic o¡spring. After identifying potentially confounding variables, further analyses were restricted to children of parents without a history of mental health disorders, with a birth weight

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Evidence-based Obstetrics and Gynecology (2006) 8, 80 ^ 81 doi:10.1016/j.ebobgyn.2006.07.003

X 2500 g, who were delivered at term (X37 completed weeks), with Apgar scores 46 at 5 min after birth. MAIN OUTCOME MEASURES Relative risk (RR) adjusted for potentially confounding variables. MAIN RESULTS In total, 59% of mothers with a hyperkinetic child were smokers, as compared to 35% of control mothers. The relative risk of giving birth to a child with a hyperkinetic disorder was 2.9 (95% CI, 2.1 ^ 4.0) for mothers who smoked. After controlling for the signi¢cantly confounding factors of maternal schooling, employment status, income, cohabitant status and history of psychiatric disorder of the parents and siblings the RR was 1.9 (95% CI, 1.4 ^ 2.7). Apgar scores o7 at 5 min after birth, birth weight lower than 2500 g, gestational age o37 weeks and maternal age of 20 to 24 years were also associated with a signi¢cantly increased risk of giving birth to a child with a hyperkinetic disorder. This association remained even after restricting analysis to children weighing X2500 g, 437 weeks completed gestation and with Apgar scores X7 at 5 min after birth. CONCLUSION An increased risk of giving birth to a child with a hyperkinetic disorder was found for women who smoked during pregnancy.

Overall study quality (out of 10) Topic importance Methodological quality Practical relevance

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1361-259X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved.

ARTICLE IN PRESS Commentary Smoking during pregnancy has consistently been shown to be a risk for a variety of disorders in the offspring.1 Several reasons for this association have been suggested, including brain alterations arising from prenatal exposure to nicotine2 and genetic mediation.3 Other factors which are known to be associated with both increased risk for smoking during pregnancy and hyperkinetic disorders, such as low socio-economic status, might also account for the observed association. This study addresses a crucial health issue, and provides an important contribution to existing knowledge on this topic. The authors use a well-characterized sample, from which they were able to derive information regarding many of the potential gestational, socioeconomic and psychiatric confounding factors for the association between smoking in pregnancy and hyperkinetic disorder. Furthermore, controls were matched for birth-date and gender with the 170 diagnosed cases. The authors reported that 59% of mothers of hyperkinetic children were smokers compared with 35% of the parents of controls. This represented a 3-fold increase for the risk of a mother who smokes during pregnancy giving birth to a child with hyperkinetic disorder compared with mothers who did not smoke during pregnancy (RR ¼ 2:9; 2.1^ 4.0). The authors also highlighted numerous characteristics of the parents at the time of the child’s birth that were signif|cant confounding factors for hyperkinetic disorder. After controlling for these factors using multiple logistic regression, and excluding children who were born prior to 37 weeks of completed gestation, those with low birth weight (o 2500 g), and those with Apgar scores below 7 at 5 min postpartum, the association remained, albeit with a slightly smaller magnitude (RR ¼ 1:9; 1.3^2.8). The authors also noted that the association remained after controlling for some psychiatric disorders, providing somewhat limited evidence that the association is not wholly due to genetic transmission of an underlying disorder.

The study did suffer from some limitations. In particular, maternal smoking during pregnancy was dichotomized into smoking and non-smoking. Consequently, the authors neglected to consider either a dose-response relationship or the differential impact of smoking at different stages of pregnancy. To conclude, the authors contributed to existing evidence that smoking during pregnancy increases the risk of giving birth to a child with a hyperkinetic disorder. Consequently, pregnant women should be educated about the risks to their unborn child, and encouraged to cease smoking whilst pregnant. However, many people are already aware of the dangers of smoking, both to their own health as well as that of their child, and yet continue to smoke. As such, current methods of education and encouragement may be futile, and alternative methods to eliminate smoking, especially in pregnant mothers, need to be identif|ed. Tanya M.M. Button, Ph.D Institute for Behavioral Genetics, University of Colorado, Boulder, USA

Literature cited 1. Mick E, Biederman J, Faraone SV, Sayer J, Kleinman S. Case-control study of attention-def|cit hyperactivity disorder and maternal smoking, alcohol use, and drug use during pregnancy. J Am Acad Child Adolesc Psychiatry 2002 April; 41(4):378 ^ 85. 2. Cornelius MD, Day NL. The effects of tobacco use during and after pregnancy on exposed children. Alcohol Res Health 2000; 24(4): 242^9. 3. Fergusson DM, Woodward LJ, Horwood LJ. Maternal smoking during pregnancy and psychiatric adjustment in late adolescence. Arch Gen Psychiatry 1998 August; 55(8):721^7.

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