Placenta
19.1 Placenta Previa
Description and Clinical Features
Placenta previa refers to a placenta extending to or covering the internal cervical os. When there is a placenta previa at the time of delivery, a vaginal delivery would put the mother and fetus at risk of life-threatening bleeding, and hence, cesarean delivery is indicated.
A variety of terms have been used to classify subtypes of placenta previa, including: (i) complete previa: one that covers the internal os entirely; (ii) marginal previa: one that extends to the edge of, but does not cover, the internal os; (iii) partial previa: one that partially covers the internal os (which can only occur if the internal os is dilated). The terminology, however, has been used inconsistently at times, with some authors using “partial previa” as synonymous with “marginal previa.”
An updated approach to terminology has been recommended by a multisociety workshop that took place in 2012. “Placenta previa” refers to a placenta that covers the internal cervical os, and “low-lying placenta” refers to a placenta that extends to within 2 cm of the internal os. A low-lying placenta in the second or early third trimester frequently resolves by the mid-to-late third trimester.
Sonography
Placenta previa can be diagnosed by transabdominal or transvaginal sonography. With either of these scanning techniques, a complete previa should be diagnosed when the placenta covers the internal cervical os (Figure 19.1.1), and a low-lying placenta should be diagnosed if the placenta extends to within 2 cm of the os (Figure 19.1.2).
Transabdominal sonography is the primary approach to diagnosing placenta previa and should be performed with the bladder partially full. An empty bladder can make visualization of the relevant area difficult, and an overly full bladder can simulate a previa (pseudo-previa) by causing apposition of the anterior and posterior walls of the lower uterine segment. If the lower segment is obscured by the presenting fetal part, manual elevation of the fetus by abdominal palpation is often helpful (Figure 19.1.3). If the presenting part cannot be elevated, transvaginal scanning (Figure 19.1.4) can then be used to assess for previa.
Another pitfall in the diagnosis of placenta previa is when a lower uterine segment contraction is present, distorting the placenta and myometrium to make it appear as though the placenta covers the cervix. In this situation, waiting 10–20 minutes to allow the contraction to resolve will permit the correct diagnosis of previa or no previa (Figure 19.1.5).
 Figure 19.1.1 Complete placenta previa. A: Sagittal transabdominal midline view of the lower uterus demonstrates the placenta (PL) covering the internal os (arrowhead) of the cervix (CX). B: Transvaginal midline view of the lower uterus and cervix (CX) in another patient demonstrating the placenta (PL) covering the internal os (arrowhead). |
 Figure 19.1.2 Low-lying placenta. Sagittal views of the cervix and lower uterine segment done transabdominally (A) and transvaginally (B) demonstrate the placenta (PL) with its edge extending to within 2 cm of the internal cervical os (calipers). |
 Figure 19.1.3 Low-lying placenta demonstrated by manually lifting the fetal head. A: On this sagittal view of the lower uterine segment, the fetal head (HD) casts an acoustic shadow (SH) that obscures the posterior lower uterine segment and a part of the cervix (arrows). A small echogenic structure (arrowhead) that extends caudal to the shadow is suspicious, but not definitive, for a low-lying placenta. B: Sagittal view of the lower uterine segment after manually lifting the fetal head definitively demonstrates a low-lying placenta, with the placental edge (long arrow) partially covering the cervix (short arrows) and ending close to the internal os (arrowhead). |
 Figure 19.1.4 Fetal head obscuring low-lying placenta. A: Sagittal midline transabdominal view of the lower uterus demonstrates the fetal head (HD) overlying the cervix (CX), which does not permit assessment of placental location beneath the head. An attempt was made to manually lift the head, but the head could not be moved so a transvaginal scan (B) was performed. This demonstrated a low-lying placenta, with the placental edge less than 1 cm from the internal os (calipers). |
 Figure 19.1.5 Lower uterine segment contraction does not permit diagnosis of placenta previa. A: Sagittal midline view of the lower uterus demonstrates a contraction (*’s) in the lower uterine segment. The placenta (PL) appears to overlie the cervix (CX). B: After the contraction resolves, the placenta (PL) is seen to end above the cervix (CX). C: Sagittal midline view of the lower uterus in another patient demonstrates a contraction (*’s) in the lower uterine segment. The placenta (PL) appears to overlie the cervix (CX), similar to the appearance in the patient in (A). D: After the contraction resolves, the placenta (PL) is seen to end very close to the internal os (arrowhead) of the cervix (CX), indicating a low-lying placenta. |
19.2 Placental Abruption
Description and Clinical Features
Placental abruption refers to separation of part or all of the placenta from the uterine wall prior to delivery. The mother often presents clinically with pain and bleeding but may be asymptomatic. Abruption can lead to fetal morbidity or death from hypoxia or exsanguination, and hence, rapid and accurate diagnosis of abruption can be critical to pregnancy management.
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