Self-selection and bias in a large prospective pregnancy cohort in Norway

Department of Public Health and Primary Health Care, University of Bergen, Kalfarveien 31, N-5018 Bergen, Norway.
Paediatric and Perinatal Epidemiology (Impact Factor: 3.13). 11/2009; 23(6):597-608. DOI: 10.1111/j.1365-3016.2009.01062.x
Source: PubMed


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.

11 Reads
  • Source
    • "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). "
    [Show abstract] [Hide abstract]
    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
  • Source
    • "Non-participation was higher among pregnant women who were <25 years, living alone, smokers, and who had >2 previous births or previous stillbirths. MoBa mothers had somewhat higher education, a lower prevalence of preterm births and fewer babies with low birth weights compared with the general population of pregnant women in Norway in 1999 to 2008 [32]. Follow-up is still on-going through questionnaires at regular intervals and by linkage to national health registries. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Maternal risk drinking may be a risk factor for child behavior problems even if the mother has discontinued this behavior. Whether pre-pregnancy risk drinking is an independent predictor of child behavior problems, or whether a potential effect may be explained by maternal alcohol use during and after pregnancy or other adverse maternal characteristics, is not known. Employing data from the Norwegian Mother and Child Cohort Study (MoBa), longitudinal associations between maternal pre-pregnancy risk drinking and behavior problems in toddlers aged 18 and 36 months were examined. Included in the study was mothers answering MoBa questionnaires when the child was 18 (N = 56,682) and 36 months (N = 46,756), and who had responded to questions regarding pre-pregnancy risk drinking at gestation week 17/18, using the screening instrument T-ACE. Toddler behavior problems were measured with items from Child Behavior Checklist. Associations were analyzed with multivariate logistic regression, controlling for pre and postnatal alcohol use, as well as other relevant covariates. Pre-pregnancy risk drinking was associated with child behavior problems at 18 and 36 months, even after controlling for pre and postnatal alcohol use. Maternal ADHD and anxiety and depression were the only covariates that had any substantial impact on the associations. When all covariates were included in the model, the associations were weak for internalizing behavior problems and non-significant for externalizing behavior problems. Pre-pregnancy risk drinking may predict early development of behavior problems in the offspring. This increased risk may be due to other adverse maternal characteristics associated with risk drinking, in particular co-occurring maternal psychopathology.
    European Child & Adolescent Psychiatry 07/2014; 23(10). DOI:10.1007/s00787-014-0588-x · 3.34 Impact Factor
  • Source
    • "Our finding that a weight gain between 5 to 28 kg corresponded to a combined predicted risk of 20% for SGA and LGA appears, however, unrealistic. One explanation might be that in our cohort SGA was only found among 7.0% of the babies born to normal weight women, which again might be explained by fewer smokers and higher education level compared to the general population from where the cut off values for SGA and LGA were computed [26]. The IOM recommendations corresponded to a joint predicted risk of about 14%, but the nadir of the curve was not in the center of the IOM recommendations, again indicating a skewed representation in our cohort. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Background Excessive gestational weight gain (GWG) is associated with pregnancy complications, and Norwegian Health Authorities have adopted the GWG recommendations of the US Institute of Medicine and National Research Council (IOM). The aim of this study was to evaluate if a GWG outside the IOM recommendation in a Norwegian population is associated with increased risk of pregnancy complications like hypertension, low and high birth weight, preeclampsia, emergency caesarean delivery, and maternal post-partum weight retention (PPWR) at 6 and 18 months. Methods This study was performed in 56 101 pregnant women included in the prospective national Norwegian Mother and Child Cohort Study (MoBa) in the years 1999 to 2008. Women who delivered a singleton live born child during gestational week 37 to 42 were included. Maternal prepregnant and postpartum weight was collected from questionnaires at 17th week of gestation and 6 and 18 months postpartum. Results A weight gain less than the IOM recommendations (GWG < IOM rec.) increased the risk for giving birth to a low weight baby among normal weight nulliparous women. A weight gain higher than the IOM recommendations (GWG > IOM rec.) significantly increased the risk of pregnancy hypertension, a high birth weight baby, preeclampsia and emergency cesarean delivery in both nulliparous and parous normal weight women. Similar results were found for overweight women except for no increased risk for gestational hypertension in parous women with GWG > IOM rec. Seventy-four percent of the overweight nulliparous women and 66% of the obese women had a GWG > IOM rec. A GWG > IOM rec. resulted in increased risk of PPWR > 2 kg in all weight classes, but most women attained their prepregnant weight class by 18 months post-partum. Conclusions For prepregnant normal weight and overweight women a GWG > IOM rec. increased the risk for unfavorable birth outcomes in both nulliparous and parous women. A GWG > IOM rec. increased the risk of a PPWR > 2 kg at 18 months in all weight classes. This large study supports the Norwegian Health authorities’ recommendations for normal weight and overweight women to comply with the IOM rec.
    BMC Pregnancy and Childbirth 06/2014; 14(1):201. DOI:10.1186/1471-2393-14-201 · 2.19 Impact Factor
Show more