Objectives To elucidate the research possibilities when merging data on maternal diet from the Danish National Birth Cohort (DNBC) and the Norwegian Mother and Child Cohort Study (MoBa), through comparison of (i) the methodology used for dietary assessment and (ii) the estimated intake of selected food groups in the two cohorts.

Design Qualitative and quantitative comparison of the two dietary databases.

Setting Two national prospective pregnancy cohorts. Population Denmark, Norway.

Methods Comparison of food intake using food frequency questionnaires (FFQs).

Results The FFQs had overlapping time windows and a majority of the questions in the two FFQs were comparable. Calculation principles shared similar features, including the software used and use of global questions to calibrate intakes of different food groups. A total of 63 food groups were defined that could be compared across the two cohorts; these were further aggregated down to 31 broader groups. A comparison of food intakes (grams/d) showed 39, 74 and 141% lower daily intakes of fish, potatoes and rice, respectively, in DNBC vs. MoBa and 39, 54 and 65% higher daily intakes of milk, butter and potatoes in DNBC vs. MoBa. For most other food groups, differences in consumption data were below 20%.

Conclusions The two FFQs are to a large extent compatible and substantial differences in dietary habits were observed between the two cohorts. This may strengthen studies using pooled analysis to examine diet-disease relations. This is a conclusion of great importance given the colossal and costly task involved to establish each of these two cohorts.