Assessment of placental location in the mid-trimester fetal anatomic scan is a critical component of sonographic examination as recommended by various national and international guidelines.1,2 It allows for timely identification of at-risk pregnancies to ensure close surveillance for optimal peripartum management that minimizes maternal and neonatal morbidity and mortality.
Due to varying criteria used at different gestational ages, the true incidence of placenta previa is difficult to determine3, and at 18 to 23 weeks it has been reported to be around 5% when evaluated by transabdominal scan and 1.5% when evaluated by transvaginal scan.4 The majority resolve with advancing gestation with a 0.5% incidence reported at term. However, prevalence of placenta previa is on the rise5 and there are several risk factors for abnormal placentation, at the forefront of which is prior uterine instrumentation, whether it be cesarean delivery, dilatation and curettage or myomectomy. With each cesarean birth, the likelihood of placenta previa in a future pregnancy increases with reported relative risks of 4.5, 7.4, 6.5 and 44.9 for one, two, three and four prior cesarean sections, respectively.6 The risk is two-fold higher in case of pre-labor cesarean section in comparison to an intrapartum cesarean section.7 In addition, there are other predisposing factors to placenta previa such as higher-order gestation, advanced maternal age, grand multiparity and pregnancies resulting from assisted reproductive technology.5,8
Besides the maternal risks of bleeding antepartum, intrapartum, and postpartum, neonates born to mothers with placenta previa, especially with pregnancy bleeding, are at a higher risk for iatrogenic prematurity as well perinatal morbidity and mortality.8 In addition, there has been a reported mild increase in intrauterine growth restriction/small for gestational age in neonates from pregnancies with a placenta previa.9
A multisociety fetal imaging forum meeting in 2014 defined a low-lying placenta as having the inferior placental edge within 2 cm from the internal os, and a placenta previa where the placenta covers the internal os (Table 1).10 The forum also recommended abandoning the terms partial and complete placenta previa.
Presence of placenta previa increases risk of placenta accreta spectrum which has added comorbidities. Sonologists should be aware of the increased risk and should take all precautions for diagnosis and proper management of their patients.8
The aim of this review is to provide practical tips on diagnosis and management of a placenta previa.
The author reports no potential conflicts of interest with regard to this article.
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