Self-selection and bias in a large prospective pregnancy cohort in Norway
ABSTRACT Self-selection in epidemiological studies may introduce selection bias and influence the validity of study results. To evaluate potential bias due to self-selection in a large prospective pregnancy cohort in Norway, the authors studied differences in prevalence estimates and association measures between study participants and all women giving birth in Norway. Women who agreed to participate in the Norwegian Mother and Child Cohort Study (43.5% of invited; n = 73 579) were compared with all women giving birth in Norway (n = 398 849) using data from the population-based Medical Birth Registry of Norway in 2000-2006. Bias in the prevalence of 23 exposure and outcome variables was measured as the ratio of relative frequencies, whereas bias in exposure-outcome associations of eight relationships was measured as the ratio of odds ratios. Statistically significant relative differences in prevalence estimates between the cohort participants and the total population were found for all variables, except for maternal epilepsy, chronic hypertension and pre-eclampsia. There was a strong under-representation of the youngest women (<25 years), those living alone, mothers with more than two previous births and with previous stillbirths (relative deviation 30-45%). In addition, smokers, women with stillbirths and neonatal death were markedly under-represented in the cohort (relative deviation 22-43%), while multivitamin and folic acid supplement users were over-represented (relative deviation 31-43%). Despite this, no statistically relative differences in association measures were found between participants and the total population regarding the eight exposure-outcome associations. Using data from the Medical Birth Registry of Norway, this study suggests that prevalence estimates of exposures and outcomes, but not estimates of exposure-outcome associations are biased due to self-selection in the Norwegian Mother and Child Cohort Study.
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- "Demographic, health-, pregnancy-and birth-related variables have previously been examined to investigate potential self-selection bias in MoBa. Despite risk prevalence differences between the sample and the population , estimates of risk exposure and child developmental outcomes were not significantly different when MoBa participants were compared with the entire population of Norwegian mothers (Nilsen et al. 2009). "
ABSTRACT: Background Previous research has found that language and motor skills are closely interrelated developmental areas. This observation has led to questions about the specificity of these domains, and the nature of the associations. In this study, we investigated the longitudinal relationship between language and gross and fine motor performance from 3 to 5 years of age. Methods We tested the prediction across and within developmental domains using cross-lagged panel models. In addition, estimates of specificity for each domain were calculated. Analyses were performed using parental reports in a sample of 11 999 children from a prospective population study. Results Structural equation modelling revealed unique positive predictions from early language performance to later fine and gross motor skills. Neither gross nor fine early motor skills uniquely predicted later language performance. Both language and motor skills were stable from 3 to 5 years of age. Motor skills were more stable in boys than in girls. Boys had lower scores than girls on fine motor performance, but gender differences in cross-lagged associations between language and motor performance were non-significant. The variance specific to language performance decreased from 68% to 46% in relation to fine motor skills and from 61% to 46% in relation to gross motor skills from 3 to 5 years of age. Conclusion From 3 to 5 years of age the stability within each developmental area is high, and unique prediction from one domain to the other is weak. These results implicate stable and correlated developmental pathways at this age.08/2014; 2(34). DOI:10.1186/s40359-014-0034-3
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- "The present study was based on the Norwegian Mother and Child Cohort Study (MoBa) conducted by the Norwegian Institute of Public Health (Magnus et al. 2006; Nilsen et al. 2009). The MoBa study has been following pregnant women from the 17th week of pregnancy to the children's 3rd birthday, with recruitment starting in 1999 and ending in 2008. "
ABSTRACT: This prospective population-based study examined associations between children's behaviour problems and maternal employment. Information on children's behaviour problems at 3 years from 22,115 mothers employed before pregnancy and participating in the Norwegian Mother and Child Cohort Study were linked to national register data on employment and relevant social background factors, mothers' self-reported susceptibility to anxiety/depression and mother-reports of day-care attendance and fathers' income. Mothers reporting their child to have severe (>2 SD) internalizing or severe combined behaviour problems (5 %) had excess risk of leaving paid employment irrespective of other important characteristics generally associated with maternal employment (RR 1.24-1.31). The attributable risk percent ranged from 30.3 % (internalizing problems) to 32.4 % (combined problems). Externalizing behaviour problems were not uniquely associated with mothers leaving employment.Journal of Family and Economic Issues 10/2013; DOI:10.1007/s10834-013-9378-8
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- "However, the relationships between exposure and outcomes are not necessarily distorted by exclusion of non-participants. For example, for eight exposure–disease relationships examined in the MoBa data, the associations were essentially the same as those found in an analysis of data for the entire pregnant population in Norway (Nilsen et al., 2009). The findings in this study will form the basis for on-going and future studies in the MoBa examining prenatal exposure to dioxins and PCBs and birth outcomes (Papadopoulou et al., unpublished results) as well as the future health of the children of these pregnant women. "
ABSTRACT: Exposure to dioxins and polychlorinated biphenyls (PCBs) during pregnancy and breastfeeding may result in adverse health effects in children. Prenatal exposure is determined by the concentrations of dioxins and PCBs in maternal blood, which reflect the body burden obtained by long term dietary exposure. The aims of this study were (1) to describe dietary exposure and important dietary sources to dioxins and PCBs in a large group of pregnant women and (2) to identify maternal characteristics associated with high dietary exposure to dioxins and PCBs. Dietary exposure to dioxins (sum of toxic equivalents (TEQs) from dioxin-like (dl) compounds) and PCB-153 in 83,524 pregnant women (gestational weeks 17-22) who participated in the Norwegian Mother and Child Cohort Study (MoBa) during the years 2002-2009 was calculated based on a food frequency questionnaire (FFQ) and a database of dioxin and PCB concentrations in Norwegian food. The median (interquartile range, IQR) intake of PCB-153 (marker of ndl-PCBs) was 0.81 (0.77) ng/kg bw/day. For dioxins and dioxin-like PCBs, the median (IQR) intake was 0.56 (0.37) pg TEQ/kg bw/day. Moreover, 2.3% of the participants had intakes exceeding the tolerable weekly intake (TWI) of 14pg TEQ/kg bw/week. Multiple regression analysis showed that dietary exposure was positively associated with maternal age, maternal education, weight gain during pregnancy, being a student, and alcohol consumption during pregnancy and negatively associated with pre-pregnancy BMI and smoking. A high dietary exposure to PCB-153 or dl-compounds (TEQ) was mainly explained by the consumption of seagull eggs and/or pate with fish liver and roe. Women who according to Norwegian recommendations avoid these food items generally do not have dietary exposure above the tolerable intake of dioxins and dl-PCBs.Environment international 08/2013; 59C:398-407. DOI:10.1016/j.envint.2013.07.001 · 5.66 Impact Factor