Revisiting the Effect of Maternal Smoking during Pregnancy on Offspring Birthweight: A Quasi-Experimental Sibling Analysis in Sweden

Abstract

Maternal smoking during pregnancy (SDP) seems associated with reduced birthweight in the offspring. This observation, however, is based on conventional epidemiological analyses, and it might be confounded by unobserved maternal characteristics related to both smoking habits and offspring birth weight. Therefore, we apply a quasi-experimental sibling analysis to revisit previous findings. Using the Swedish Medical Birth Register, we identified 677,922 singletons born between 2002 and 2010 from native Swedish mothers. From this population, we isolated 62,941 siblings from 28,768 mothers with discrepant habits of SDP. We applied conventional and mother-specific multilevel linear regression models to investigate the association between maternal SDP and offspring birthweight. Depending on the mother was light or heavy smoker and the timing of exposition during pregnancy (i.e., first or third trimester), the effect of smoking on birthweight reduction was between 6 and 78 g less marked in the sibling analysis than in the conventional analysis. Sibling analysis showed that continuous smoking reduces birthweight by 162 grams for mothers who were light smokers (1 to 9 cigarettes per day) and 226 g on average for those who were heavy smokers throughout the pregnancy in comparison to non-smoker mothers. Quitting smoking during pregnancy partly counteracted the smoking-related birthweight reduction by 1 to 29 g, and a subsequent smoking relapse during pregnancy reduced birthweight by 77 to 83 g. The sibling analysis provides strong evidence that maternal SDP reduces offspring birthweight, though this reduction was not as great as that observed in the conventional analysis. Our findings support public health interventions aimed to prevent SDP and to persuade those who already smoke to quit and not relapse throughout the pregnancy. Besides, further analyses are needed in order to explain the mechanisms through which smoking reduces birthweight and to identify other maternal characteristics that are common causes of both birthweight reduction and maternal smoking.

Evaluation of Health in Pregnancy grants in Scotland: a natural experiment using routine data

“…Smoking during pregnancy reduces birthweight by 162-377 g, depending on daily consumption (there is a greater reduction for heavy smokers) and the trimester in which exposure occurs (a greater reduction is seen during the third trimester). 24,26,27 A number of interventions have been developed to improve pregnancy outcomes, 28 but evidence of the success of any interventions is, in most cases, still limited. An analysis of the potential to reduce preterm birth rates in high-income countries focused solely on five interventions for which there were high levels of evidence: (1) smoking cessation; (2) progesterone; (3) cervical cerclage; (4) decrease in non-medically indicated caesarean delivery and induction; and (5) a limit on multiple embryo transfer in assisted reproductive technology.…”

Birth weight differences between those offered financial voucher incentives for verified smoking cessation and control participants enrolled in the Cessation in Pregnancy Incentives Trial (CPIT), employing an intuitive approach and a Complier Average Causal Effects (CACE) analysis

“…Smoking during pregnancy, as a risk factor on its own, will result in a baby born 160 g smaller to women who continue to smoke up to nine cigarettes per day and 230 g smaller to those who continue to smoke more than nine cigarettes per day) [8]. …”

Month-Wise Estimates of Tobacco Smoking During Pregnancy for the United States, 2002–2009

“…The timing of prenatal tobacco smoking exposure can be crucial for pregnancy outcomes and fetal development, with potential smoking-affected epigenetic processes as early as the embryonic period of development and before Month 2 of pregnancy has ended (1-4). Low birth weight (LBW) provides one illustration of timing issues.…”