Comparison of risk factors for placental abruption and placenta previa: Case‐cohort study

AIM: A case-cohort study was performed to clarify and compare the risk factors for placental abruption and placenta previa. MATERIAL & METHODS: This study reviewed 242,715 births at 125 centers of the perinatal network in Japan from 2001 through to 2005 as a base-cohort. Women with singleton pregnancies delivered after 22 weeks of gestation were included. The evaluation determined the risk factors for placental abruption and placenta previa. Five thousand and thirty-six births (2.1%) were determined as the subcohort by random selection. Acute-inflammation-associated clinical conditions (premature rupture of membranes and clinical chorioamnionitis) and chronic processes associated with vascular dysfunction or chronic inflammation (chronic and pregnancy-induced hypertension, pre-existing or gestational diabetes and maternal smoking) was examined between the two groups. RESULTS: Placental abruption and placenta previa were recorded in 10.1 per 1000 and 13.9 per 1000 singleton births. Risk factors for abruption and previa, respectively, included maternal age over 35 years (adjusted risk ratios [RRs]=1.20 and 1.78), IVF-ET (RRs = 1.38 and 2.94), preterm labor (RRs = 1.63 and 3.09). Smoking (RRs = 1.37), hypertension (RRs = 2.48), and pregnancy-induced hypertension (RR = 4.45) were risk factors for abruption but not for previa. On the other hand, multiparity (RR= 1.18) was a risk factor for previa but not for abruption. The rates of acute-inflammation-associated conditions and chronic processes were higher among women with abruption than with previa. (RR 2.0 and 4.08, respectively). CONCLUSION: The case-cohort study technique elucidated the difference in the risk factors for placental abruption and placenta previa.