Our study provides an overview of calcium intake levels by pregnant women based on not just diet but also supplement use. A major strength of our study is the large sample size of 2477 participants. In our population, women of Caucasian origin were somewhat overrepresented and more than half of the population had a high educational level, which corresponds to the composition of the population of Dutch women in their thirties [
43]. A previous Canadian study showed that calcium intake was lower in women with lowest educational levels [
40], so the overrepresentation of high educated women in our study may have led to an underestimation of the total percentage of women with an inadequate intake.
A few limitations of our study should be addressed. First, since there is no biochemical assay to display the nutritional calcium status, we had to depend on questionnaires. Repeated dietary recalls or records might have been considered as more accurate approaches for food intake assessment. However, this method would not be achievable in a large cohort. Nevertheless, the FFQ method is widely used for food product and nutrient intake assessment, and although its main strength is in the ranking of individuals according to their intakes of frequently used foods and nutrients, it is also considered a feasible tool to gain insight in the percentage of inadequate intake in a large population [
44]. Second, our assessment of dietary calcium intake included only those products with the highest contribution to their calcium content to minimize the load of the questionnaire. Moreover, the selected products inquired in our questionnaire contributed to more than 60% of all dietary calcium intake and we recalculated total calcium intake to 100% [
45]. This selection procedure probably has resulted in an underestimation of dietary calcium intake, requiring adjustment [
40]. The five most contributing food products to dietary calcium intake that were not inquired in our study were bread (3.9%), water (3.7%), cooked or stir fried vegetables (3.5%), coffee (2.2%), and tea (1.6%). Even though the contribution of these food products to total calcium intake is limited and already covered in the recalculation from 61.65 to 100%, we compared our methods to the methods used in a more extensive FFQ. To ensure that our methods estimated the correct total calcium amount, we applied our methods to the data of pregnant women from an older birth cohort study (
N = 2855) [
46]. Thirty-nine from the 213 food items from the KOALA-FFQ were comparable to the food items which we inquired in our study. Total calcium intake based on the 39 food items was strongly correlated to calcium intake based on the complete KOALA-FFQ (Pearson’s
r = 0.95). Women who did not consume food products inquired in our questionnaire were contacted and have a recalculated total dietary calcium intake of 0 mg, while they may have consumed other calcium-containing products. However, sensitivity analysis did not show large differences after exclusion of outliers. Third, recall may not have been optimal as dietary intake was inquired for the month prior to filling in the questionnaire. Finally, there may have been intra-individual variation in food intake which was not covered by the applied measurement procedure. Food intake may vary over time and, perhaps, especially during pregnancy as women may experience sickness in the early pregnancy. However, the previous evidence showed that calcium intake from supplement use does not differ much across trimesters [
47].