Undiagnosed abnormal postpartum blood loss: Incidence and risk factors.

We aimed to evaluate the incidence of undiagnosed abnormal postpartum blood loss (UPPBL) after vaginal delivery, identify the risk factors and compare them to those of postpartum haemorrhage (PPH).The study population included women who participated in a randomized controlled trial of women with singleton low-risk pregnancy who delivered vaginally after 35 weeks' gestation (n = 3917). Clinical PPH was defined as postpartum blood loss ≥ 500 mL measured by using a collector bag and UPPBL was defined by a peripartum change in haemoglobin ≥ 2 g/dL in the absence of clinical PPH. Risk factors were assessed by multivariate multinomial logistic regression.The incidence of UPPBL and PPH was 11.2% and 11.0% of vaginal deliveries, respectively. The median peripartum change in Hb level was comparable between UPPBL and PPH groups (2.5 g/dL interquartile range [2.2-3.0] and 2.4 g/dL IQR [1.5-3.3]). Risk factors specifically associated with UPPBL were Asian geographical origin (adjusted OR [aOR] 2.3, 95% confidence interval [CI] 1.2-4.2; p = 0.009), previous caesarean section (aOR 3.4, 2.1-5.5; p<0.001) and episiotomy (aOR 2.6, 1.8-3.6; p