Placental weight and excess postpartum haemorrhage: a population study of 308 717 pregnancies

Objective To investigate whether placental weight may be positively associated with the prevalence of excess postpartum haemorrhage because large placentas have large surface areas. Design Registry-based cross-sectional study. Setting Population study. Population All singleton deliveries after 21 weeks of gestation in Norway during 1999-2004 (n = 308 717). Methods Data were obtained from the Medical Birth Registry of Norway, which is based on compulsory notification of births by the midwife or doctor in charge of the delivery. Main outcome measure Excess postpartum haemorrhage was defined as bleeding of 500 ml or more within 2 hours of delivery. Results There was a gradual increase in the prevalence of excess postpartum haemorrhage with increasing placental weight (test for trend, P < 0.05). Having a placenta of 1100 g or more was associated with 2.5 times (odds ratio 2.54, 95% CI 2.31-2.79) higher prevalence than having a placenta of 300-499 g, after adjustment for offspring birthweight, parity, caesarean section and placenta-related and delivery-related complications. A large placenta relative to birthweight was also associated with higher prevalence of excess postpartum haemorrhage. Conclusion The size of the placental surface may explain the positive association of placental weight with the prevalence of postpartum haemorrhage. In pregnancies with a large placenta relative to offspring birthweight, other factors that enhance bleeding may also play a role.