Objectives The MoBa study includes data from mothers on several occasions during pregnancy and after birth. The purpose of this study was to compare education level and mid-pregnancy sickness absence in self-report MoBa data and in national registries.

Methods We followed MoBa participating mothers, born in Norway 1967–1976, in several national registers. The first pregnancy 1999–2009 to 36 629 mothers was considered for analysis of education level in MoBa and the National Education Database, and for mid-pregnancy sickness absence from work in MoBa and in the national event database FD-Trygd. The degree of agreement in 5-level education level and dichotomous absence >14 days between gestational weeks 13 and 30 were estimated in tabular analyses. We computed associations between sickness absence and years of education in binomial regression as a measure of educational gradient in sickness absence.

Results The agreement in education level among 31 833 mothers with data from both sources was 70%, the self-report level was higher than the register data for 24%, register data level was higher for 6%. The main discrepancy was for master level education (25% in self-report data and 12% in registry data). The sickness absence risk was 38% in self-report data and 44% in registry data, with an 81% agreement for the 11 801 mothers with data from both sources. A one-year increment in education was associated with a 3.2 percent point risk reduction in absence both for self-report data and registry data (95% confidence intervals 2.9 to 3.7 percent points for self-report data and 2.8 to 3.5 percent points for registry data).

Conclusions Participating MoBa mothers tended to report higher education level and lower sickness absence than for data collected in national registries. This difference did not seem to influence associations between education level and sickness absence risk in mid-pregnancy.