Female twins included in this study were younger than those who were not (p<0.0001) because of older participants aging out of the study after age 18 before data collection was completed over all waves. The rates of CD were comparable between study participants (3.1%) and those not included (2.9%, p = 0.88). The average number of maternal ASP symptoms between subsample members (0.89±0.98) and those not included (0.81±0.93) did not differ significantly (p = 0.11). Individuals included in this study were similar for measured census-based indicators of socioeconomic status such as median family income (p = 0.26), rural versus urban residence (p = 0.99) and college education (p = 0.44). Thus, although the females in this study were younger than the larger sample, they represent the total sample with respect to CD prevalence and risk factor exposure.
