Placenta previa occurs when the placenta is partially or completely covering the internal os (cervix). It is diagnosed via ultrasound between 18-24 week of gestation and occurs in approximately 3.9% of pregnancies. In most cases however, the placenta will move upwards and out of the way of the cervix reducing the risk to 1.9% at term. If the overlap in the second trimester is greater than 15mm there is an increased chance that a caesarean will be needed; if greater than 25mm the risk is 40-100%. A watch and wait approach is taken at this time and your midwife or doctor may require you have further ultrasounds to confirm the placental location throughout pregnancy. Depending on the degree of previa you may be put on bed rest to prevent damaging the placenta as your baby grows.
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Source: Oppenheimer, L., et al. 2007. Diagnosis and management of placenta previa. JOGC 189:261-266.