Maternal smoking during pregnancy and scholastic achievement in childhood: evidence from the LIFECOURSE cohort study

Abstract

Introduction

Existing recommendations state that pregnant women should not use tobacco products due to the toxic impact on the fetus,¹ negative influence on birth weight,^2,3 and increased risks for preterm birth.⁴ Maternal smoking during pregnancy (MSDP) has also been shown to be associated with elevated risks of poor cognitive and developmental outcomes in the offspring, with study outcomes ranging in time from shortly after birth into adolescence and young adulthood. For example, MSDP has been related to lower intelligence in 9-year-old children,⁵ reduced intellectual abilities in 8-year-olds,⁶ inferior visual-motor integration and verbal competence in 4–5 year-old children,⁷ attentional deficit in children aged 5⁸ and 10,⁹ lower scholastic achievement in 3–5 year-old children,^5,10 as well as slower physical growth in 10–15 year-old children.¹¹ MSDP has also been positively related to delinquent behaviours in youth and young adults⁹ although findings on delinquency have not been consistent across study sites and age groups.¹²

However, due to conflicting findings in previous research and limited number of studies that reach into adolescence, the long-term impact of MSDP on cognitive developmental factors such as academic achievement in the offspring are not well understood.¹³ Longer term studies have both shown negative and null associations between MSDP and intellectual and academic outcomes. For example, an Australian study of almost 4300 mothers and their children found a negative association between MSDP and scores in English, Science and Mathematics at the age of 14.¹⁴ Similar results were reported among 15-year-olds in a Swedish cohort study of over 400 000 individuals born between 1983 and 1987.¹⁵ On the other hand, Gilman et al.² found no association between MSDP and academic outcomes in 7-year-olds in the U.S. National Collaborative Perinatal Project, 1959–1974 with over 50 000 participants, and a cohort study based on all births in Sweden between 1983 and 1991 (N ∼ 650 000) found full siblings that were differentially exposed to MSDP not to differ in their academic scores and mathematics proficiency at the age of 15.¹⁶ Likewise, a case-control study of French-Canadian children aged 12–18 (N ∼ 500), reported no differences between cases and controls on the relationship between MSDP and the outcomes on a battery of 33 neuropsychological and intellectual tasks.¹⁷

Another critical problem in research on the impact of MSDP on cognitive outcomes in the offspring that have been highlighted in previous studies is the inconsistency in the selection of covariates employed in statistical analyses. Some studies control for birth-related outcome variables such as birth weight, head circumference or Apgar score (e.g. Lambe et al.¹⁵) but do not include social and family variables at the time of assessment, and some studies have selected an extensive number of social developmental covariates such as SES-related variables at the time of assessment (e.g. O’Callaghan et al.¹⁴) but lack birth-related measures. As a result, it has been hypothesized that previous observations regarding the probable impact of MSDP on scholastic outcomes may have been spurious and due to certain missing covariates such as maternal education.¹⁸ Still others have called for a more general improvement and consistency in the selection of covariates in studies of this nature.^13,19

The objective of this study is to improve our understanding of the impact of MSDP on academic achievement in early to mid-adolescence. We employ data from the LIFECOURSE study, a registry data-based cohort study conducted in Reykjavik, Iceland. We assess the impact of MSDP on standardized scholastic scores in 4th, 7th and 10th grade with real-time data collected before birth, during the neonatal period, as well as during the time of scholastic observation, therefore consistently including many of the covariates that previous studies have commonly lacked and has been called for.

Methods

Results

Table 2 includes the linear regression models for maternal smoking during pregnancy on standardized academic achievement in 4th grade. Model 1 shows that children of mothers who smoked tobacco during pregnancy scored an average of 6.30 points less than children of mothers who did not smoke during pregnancy (P = 0.01) but maternal smoking before pregnancy was not related to scholastic achievement. Model 2 includes the covariates that are associated with the time of birth, both biological (e.g. birth weight, maternal age and not first child), and social factors (e.g. household income and parental cohabitation). In short, the explanatory power improves significantly between the two models [F (7, 367) = 5.08, P = 0.000] but the impact of MSDP remains mostly unchanged. Model 3 adds the covariates associated with the year of student assessment (2010) to model 2; however, the addition of these covariates do not add a significant explanation to the model [F (3, 364) = 1.50, P = 0.215] and neither do they change the impact of MSDP on academic achievement in 4th grade, which is 5.79 points less for children of mothers who smoked tobacco during pregnancy compared with children of mothers that did not (P = 0.025).

Tables 3 and ​and44 include the linear regression models for MSDP on standardized scholastic achievement in 7th and 10th grade, respectively. In short, both models paint a very similar picture as the models displayed in table 2. In 7th grade, the impact of MSDP on grades is −7.14 points in model 1 and drops to −5.98 points in the 3rd and final model while including all covariates. Explained variance increased significantly between models 1 and 2 [F (7, 377) = 5.68, P = 0.000] as well as models 2 and 3 [F (3, 374) = 3.62, P = 0.013]. In 10th grade, the impact of MSDP on grades is −8.30 points in model 1 and drops to −6.22 points in the 3rd and final model while including all covariates. Explained variance increases significantly between models 1 and 2 [F (7, 367) = 5.96, P = 0.000] but not between models 2 and 3 [F (3, 364) = 0.48, P = 0.697].

Discussion

The results of this study show a consistent negative relationship between MSDP and standardized scholastic achievement in the offspring at the time of 4th, 7th and 10th grades. In the smoking only analyses (models 1), the differences in scholastic achievement ranged from 5.3% (6.30/120 points in 4th grade students) to 6.9% (8.30 points in 10th grade students) and from 4.8% (5.79 points in 4th grade students) to 5.2% (6.22 points in 10th grade students) in the full models including all control variables and covariates. Interestingly, smoking before pregnancy was unrelated to scholastic achievement in all models. The consistency of these findings are particularly noteworthy given the large range of covariates that was selected for the analysis and are both associated with the physical birth (e.g. birth weight, maternal age at birth and sex), the social environment into which the baby is born (e.g. birth order, parental cohabitation status and total family income during the year of birth), as well as SES related variables at the year of scholastic assessment (e.g. parental cohabitation status, total family income and parental education). The findings differ from the review results by Clifford et al.¹³ where many reviewed studies showed little or no impact of MSDP on cognitive performance and observed relationships were often masked out by other factors. In addition, the homogenous nature of the Icelandic population should ideally serve to deter the observed differences in scholastic achievement as it operates as a natural environmental control measure. Iceland has been described as being universally middle-class where the lifestyle, values and beliefs tend to be alike across the population.²⁴ Iceland’s relative lack of social hierarchy therefore serves as evidence of a greater likelihood for a biological link between MSDP and scholastic outcomes in the offspring given that we were able to control for the little variation that exists. Also, the fact that smoking before pregnancy was consistently unrelated to the outcome measures lends further support to the potential biological link between MSDP and scholastic achievement in the offspring.

There are several potential biological explanations for these findings. First, cigarette smoking includes carbon monoxide which creates carboxyhemoglobin that in turn serves to decrease the oxygen carrying capacity of maternal and fetal blood and may in some cases lead to fetal hypoxia.^25–28 Also, nicotine causes vasoconstriction, which decreases placental blood flow.^26,27 Decreased oxygen flow and blood stream have been linked to low birth weights and smaller head circumference.^26,29 Low birth weights have been associated with significant neurologic consequences such as the risk for behaviour problems,^30,31 decreased intellectual capacity, hyperactivity and learning disabilities,^2,6,32,33 as well reduced brain growth in utero.^25,34 Other plausible impacts are more distal in nature but authors have also speculated that a combination of factors and compound exposure may explain the longer term impact of MSDP on cognitive outcomes in the offspring.¹³

Funding

This study was funded by the European Research Council (ERC-CoG-2014-647860). The ERC had no role in formulating the study objectives and did not impact the analyses, reporting or selection of a journal outlet.

Conflict s of interest: None declared.

References