Major obstetric haemorrhage (MOH) is the main cause of severe maternal morbidity, incidence being estimated at 4.5 per 1000 deliveries. Cases are not routinely registered in the Netherlands. The objective of this study is to quantify the degree of underreporting of MOH in a large nationwide survey of severe acute maternal morbidity in the Netherlands (LEMMoN) and to estimate the true incidence of MOH in the Netherlands. Retrospective cross-match of the LEMMoN-database with the databases of local blood transfusion laboratories in 65 of 98 hospitals in the Netherlands during a 20-month period, using the capture-recapture method was used. From 16 of 65 centres, the reported transfusion data could not be confirmed by a local obstetrician for logistical reasons. These centres were excluded leaving 49 hospitals available for final analysis. In both databases together, 1018 unique cases of MOH were identified. Underreporting to LEMMoN was 35%. Hence, the true incidence of MOH in the Netherlands is at least 6.1 instead of 4.5 per 1000 deliveries. The estimated underreporting of MOH of 35% is considerable. Underreporting is inherent to large observational multicentre studies and should be anticipated and quantified to facilitate fair comparison of epidemiologic data.