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Placenta previa by itself is a risk factor.6,7 The risk of placenta accreta in patients with placenta previa, but no previous surgery, is increased over the baseline of 4 in 100,000 and ranges from 2% to 6.3%, depending upon whether maternal age is under or over 35 years.6 Wu et al found that the likelihood ratio of placenta previa for PAD was 51.42—by far the highest of any factor studied.4
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Uterine surgery, such as a cesarean section myomectomy or reconstruction, is also a strong risk factor for placenta accreta in patients with placenta previa.4,6,7 Silver et al found that in patients with placenta previa, the risk for accreta was 3, 11, 40, and 61 and 67 times the baseline for first, second, third, fourth, and fifth or more repeat cesarean deliveries.8 Clark et al found that one prior cesarean section in patients with placenta previa increased the risk to 24% and to 67% with 4 or more cesareans (and is increased even if the placenta does not lie over the uterine scar).9 However, the risk increases 5-fold if the placenta actually lies over the scar itself,6 again probably an additive factor of lower uterine segment location typical of placenta previa in addition to a surgically disturbed area of myometrium.
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Prior placenta accreta is also a risk factor. In a group of patients with presumed placenta accreta due to difficulty removing the placenta, 16% of cases had previous events of placenta accreta.10 Prior curettage and history of abortion have been mentioned in many publications as risk factors, but in a case controlled series neither of these were actually significant.4 Gielchinsky et al found dilatation and curettage (D and C) to be a factor in the less severe cases only; in the more severe cases leading to hysterectomy, increased parity, anterior low placenta, and repeated cesarean sections were the significant risk factors.10
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For some reason maternal age also increases risk in cases of placenta previa. Maternal age over 35 can double or triple baseline risk.6
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Although most patients with placenta accreta will have a placenta previa, placenta accreta occasionally can be found in patients without placenta previa11 and in patients without previous surgery, the latter often in the posterior or fundal areas. In the series of Gielchinsky et al, 20% were primiparous women without known risk factors.10
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Before the beginning of a scan, ask all patients about previous uterine surgery. Then look at the location of the placenta to determine if it could be over the scar.