Over the 13-year period, 891 640 women were registered in the surveillance system and delivered babies. Those who enrolled in the PHC system after delivery (30 567 cases), had a missing delivery date (3546 cases) or had no Hb recorded at any time during pregnancy (2722 cases) were excluded from the present study. Another 390 470 women were excluded because they were missing a recorded Hb value in one of the three trimesters. After exclusions, a total of 467 057 women (52 % of the original group) remained in the analysis. Compared with women who were excluded, the absolute difference in prevalence in the first, second and third trimesters was 3?9%, 1?6 % and 0?1 %, respectively