Objective The Women’s Health Study was designed to examine associations between maternal and paternal reproductive histories, behavioural and lifestyle risk factors and miscarriage. The objective of this study was to examine underlying differences in characteristics and pregnancy outcome between responders and non-responders.

Methods A cohort study was conducted in a large, tertiary hospital (8,500 deliveries per annum) in the Republic of Ireland in 2012. Women were randomly selected at their first booking visit (10–14wks) and were asked to complete a detailed lifestyle postal questionnaire containing common risk factors for miscarriage. Basic demographic data and pregnancy status at 20 weeks gestation were collected for all recruited women. Chi-square tests were performed to assess differences in characteristics and pregnancy status.

Results 715 women agreed to participate in the study of which 61.3% (n = 431) completed the detailed questionnaire. Responders and non-responders were similar in terms of the proportions who were primiparous (39.1% vs. 38.5%) and multiparous (25.0% vs. 21.1%). Average age was the same (32.8 (5.6) versus 32.1 (5.3) years; p = 0.441). Non-responders were slightly more likely to be nulliparous (40.5% vs. 35.9%; P = 0.256) and married (65.4% vs. 62.6%). In terms of the outcome, non-responders had a higher rate of miscarriage (40.4% vs. 32.1%; p = 0.025).

Conclusions While basic demographics were broadly similar between responders and non-responders, the discrepancy between miscarriage rates may be a result of unobserved factors. Participation bias towards healthier individuals is frequent in clinical and public health studies, and thus subsequent results should be interpreted with caution.