Objective The objective was to investigate prevalence, estimate risk factors, and antenatal suspicion of abnormally invasive placenta (AIP) associated with laparotomy in women in the Nordic countries.Design Population-based cohort study.Setting and population A 3-year Nordic collaboration among obstetricians to identify and report on uterine rupture, peripartum hysterectomy, excessive blood loss, and AIP from 2009 to 2012 The Nordic Obstetric Surveillance Study (NOSS).Methods In the NOSS study, clinicians reported AIP cases from maternity wards and the data were validated against National health registries.Main outcome measures Prevalence, risk factors, antenatal suspicion, birth complications, and risk estimations using aggregated national data.Results A total of 205 cases of AIP in association with laparotomy were identified, representing 3.4 per 10 000 deliveries. The single most important risk factor, which was reported in 49% of all cases of AIP, was placenta praevia. The risk of AIP increased seven-fold after one prior caesarean section (CS) to 56-fold after three or more CS. Prior postpartum haemorrhage was associated with six-fold increased risk of AIP (95% confidence interval 3.7-10.9). Approximately 70% of all cases were not diagnosed antepartum. Of these, 39% had prior CS and 33% had placenta praevia. ConclusionOur findings indicate that a lower CS rate in the population may be the most effective way to lower the incidence of AIP. Focused ultrasound assessment of women at high risk will likely strengthen antenatal suspicion. Prior PPH is a novel risk factor associated with an increased prevalence of AIP.Keywords Incidence, placenta accreta, prenatal diagnosis, risk factors.Tweetable abstract An ultrasound assessment in women with placenta praevia or prior CS may double the awareness for AIP.

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