Patricia Schreuder

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

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.

Cohort Profile Update: The Norwegian Mother and Child Cohort Study (MoBa)

This is an update of the Norwegian Mother and Child Cohort Study (MoBa) cohort profile which was published in 2006. Pregnant women attending a routine ultrasound examination were initially invited. The first child was born in October 1999 and the last in July 2009. The participation rate was 41%. The cohort includes more than 114 000 children, 95 000 mothers and 75 000 fathers. About 1900 pairs of twins have been born. There are approximately 16 400 women who participate with more than one pregnancy. Blood samples were obtained from both parents during pregnancy and from mothers and children (umbilical cord) after birth. Samples of DNA, RNA, whole blood, plasma and urine are stored in a biobank. During pregnancy, the mother responded to three questionnaires and the father to one. After birth, questionnaires were sent out when the child was 6 months, 18 months and 3 years old. Several sub-projects have selected participants for in-depth clinical assessment and exposure measures. The purpose of this update is to explain and describe new additions to the data collection, including questionnaires at 5, 7, 8 and 13 years as well as linkages to health registries, and to point to some findings and new areas of research. Further information can be found at [www.fhi.no/moba-en]. Researchers interested in collaboration and access to the data can complete an electronic application available on the MoBa website above.

Maternal smoking and birthweight: effect modification of period, maternal age and paternal smoking

Objective. To study the effect on birthweight of maternal smoking, and its modification by study period, maternal age and paternal smoking.

Secular trends in breastfeeding and parental smoking

To explore the association between smoking and breastfeeding, we obtained data from a retrospective questionnaire‐based national survey comprising a random sample (n= 34 799) of all mothers giving birth in Norway 1970‐91. Variables studied were postpartum smoking habits for both parents, duration of breastfeeding, infants year of birth and parental age. The response rate was 70% (n= 24 438). During the study period, the maternal postpartum smoking prevalence decreased from 38% to 26%. The proportion breastfeeding at 6 months increased from 15% to 44% among smokers, and from 30% to 72% among non‐smokers. In spite of a considerable increase in breastfeeding both among smokers and non‐smokers, the proportion of breastfeeding, non‐smoking women at 6 months was twice that of smoking women during the whole period. Furthermore, the duration of breastfeeding was shorter among young mothers and when the fathers were smoking. There was epidemiological evidence that the effect on breastfeeding of smoking might represent both biological and social mechanisms.

A biobank of primary teeth within the Norwegian Mother and Child Cohort Study (MoBa): a resource for the future.

The Norwegian Mother and Child Cohort Study (MoBa) is a prospective population-based cohort study including >100 000 pregnancies and following the children through childhood, using questionnaires and collecting biological samples. The aim of MoBa is to test specific aetiological hypotheses by estimating the association between exposure and disease, aiming at prevention. A biobank for exfoliated primary teeth collected from the children participating in MoBa has been established (MoBaTooth Biobank). Samples of tooth tissues from the primary dentition can give information about exposure to toxic and essential elements during fetal life and early infancy. Prenatally and postnatally formed tooth tissues permanently document early exposures unlike other biomarkers, as teeth form incrementally at a known rate. Results from tooth analyses will be coupled with corresponding information in the multiple questionnaires and data from analysis of other biological samples collected by MoBa. Invitations to donate one or more teeth are sent to all mothers/children in the period 2008-2016, when the child is 6.75 years old. By August 2011, 7400 participants had been recorded into the MoBaTooth database donating altogether 9798 teeth (1.3 teeth per child). The response rate was 24%, but there is a lag of >1 year in the response. Data from the tooth biobank can supply MoBa with important additional information on the uptake of trace elements during fetal life and early infancy. This information can illuminate possible causal factors of health and disease in the future.