Introduction There is some controversy in the literature regarding the outcomes of pregnancies complicated by antepartum bleeding of unknown origin (ABUO).

Objective To explore the risk of adverse maternal and perinatal outcomes in women with ABUO occurring after the first trimester of pregnancy.

Methods Cohort study based on data extracted from the Aberdeen Maternity and Neonatal Databank. The study population was all primigravidae delivering in Aberdeen Maternity Hospital between 1976 and 2010. Exposure was antepartum haemorrhage occurring after the first trimester not attributable to placenta praevia or abruption. Data were analysed using univariate and multivariate statistical methods.

Results Between 1976 and 2010, there were 7,517 women with ABUO and 68,423 women without. Women with ABUO were more likely to be smokers, belong to lower social class and have slightly higher body mass index. Multivariate analysis revealed that non-specific APH was a significant risk factor for induced labour (OR = 1.23, 95% CI = 1.16, 1.31), preterm delivery (OR = 2.30, 95% CI = 2.11, 2.50), postpartum haemorrhage (OR = 1.15, 95% CI = 1.06, 1.25), Apgar score less than 7 at 1 minute (OR = 1.12, 95% CI = 1.05, 1.21), and at 5 minutes (OR = 1.25, 95% CI = 1.04, 1.50). There was no significant association detected with preeclampsia, mode of delivery or perinatal death.

Conclusion Pregnancies complicated by ABUO are at greater risk of delivery related and neonatal adverse outcomes attributable to preterm birth, some of which is iatrogenic.