Maternal characteristics and pregnancy outcomes in the NICE birth cohort: an assessment of self-selection bias

Abstract

Background

Prospective birth cohorts are essential for identifying associations between exposures and outcomes. However, voluntary participation introduces a potential bias due to self selection since the persons that chose to participate may differ in background characteristics and behaviors.

Objectives

To investigate potential bias due to self-selection in the Nutritional impact on Immunological maturation during Childhood in relation to the Environment (NICE) birth cohort in northern Sweden.

Methods

Women in the NICE birth cohort (N = 621) were compared to nonparticipating pregnant women in Norrbotten County in northern Sweden who were eligible for participation (N = 4976) regarding maternal characteristics and lifestyle. Maternal characteristics and pregnancy outcomes were compared between the groups and associations between exposures (smoking, folic acid, BMI, parity, education) and pregnancy outcomes (birth weight and gestational age) were analyzed by linear regression analyses, examining any interaction with the group.

Results

NICE participants were more highly educated, older and more likely to cohabit than the non-participants. They more often took folic acid and multivitamin supplements and less often smoked during early pregnancy. Pregnancy outcomes (mode of delivery, gestational age at delivery, birth weight and APGAR score) did, however, not differ significantly between participants and non-participants. Smoking, BMI, education and parity affected gestational age and birth weight, but the associations were of similar magnitude in participants and non-participants, with no significant effect on the group.

Conclusion

Self-selection to the NICE study was evident in some factors related to lifestyle and socioeconomic characteristics but did not appear to skew pregnancy outcomes or alter well-known effects of certain lifestyle parameters on pregnancy outcomes.

Keywords:

• NICE birth cohort
• enrollment bias
• pregnancy cohort
• self-selection
• self selection bias

Acknowledgments

We thank all personnel at the maternity clinics, antenatal clinics and delivery ward for informing and recruiting patients. We also thank all the families who participated in the NICE cohort.

Authors’ contribution

All the authors participated in the planning and execution of this study and approved the final version. LEÖ, BJ and MB conceived the study. FM and RL extracted data from records. LEÖ and MB wrote the first draft of this manuscript. LEÖ, MB, DM, SN and RL conducted the statistical analyses. All the authors revised several versions of this manuscript and approved the final version for publication.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

The dataset used in this study originates from the Nutritional impact on Immunological maturation during Childhood in relation to the Environment (NICE) cohort.

Additional information

Funding

Funding has been received from the Swedish Research Council Formas (project no. 2018-02275), the Swedish Research Council (VR; project no 521-2013-3154 and 2019-01317), Swedish Research Council for Health, Working Life and Welfare (FORT E; 2014-0923 and 2018-00485 and Jane and Dan Olsson Foundation (2020–23). The funding sources were not involved in the study design, the collection, analysis or interpretation of data, in the writing of the article, or in the decision to submit the article for publication.