Expert review
Obstetrics
Using ultrasound in the clinical management of placental implantation abnormalities

https://doi.org/10.1016/j.ajog.2015.05.059Get rights and content

Placental implantation abnormalities, including placenta previa, placenta accreta, vasa previa, and velamentous cord insertion, can have catastrophic consequences for both mother and fetus, especially as pregnancy progresses to term. In these situations, current recommendations for management usually call for an indicated preterm delivery even in asymptomatic patients. However, the recommended gestational age(s) for delivery in asymptomatic patients are empirically determined without consideration of the recent literature regarding the usefulness of specific ultrasound findings to help individualize management. The purpose of this article is to propose literature-supported guidelines to the current opinion-based management of asymptomatic patients with placental implantation abnormalities based on relevant and specific ultrasound findings such as cervical length, distance between the internal cervical os and placenta, and placental edge thickness.

Section snippets

PIA identification

The ultrasound identification of a PIA usually starts with the second-trimester fetal anatomic scan, which is most optimally performed at 18-22 weeks. One exception is a cesarean scar pregnancy, which can be detected in first-trimester ultrasound examinations.24, 25, 26 If at any time during gestation there is suspicion for placenta previa, low-lying placenta, or difficulty in transabdominal visualization of the entire placenta, a transvaginal examination may be considered for an accurate

Placenta previa with or without placenta accreta

The most common risk factor for placenta previa is when at the second-trimester ultrasound screening examination the internal cervical os is found to be covered completely or partially by placenta. Another risk factor for placenta previa is a history of ≥1 cesarean deliveries.1, 27, 28 Ananth et al27 reported relative risks for placenta previa of 4.5, 7.4, 6.5, and 44.9 for 1, 2, 3, and ≥4 previous cesarean deliveries. This translates to an exponential increase in the risk of placenta previa

Marginal/low-lying placenta

The definition of marginal/low-lying placenta is based on the relationship between the internal cervical os and the placental edge as determined by transvaginal ultrasound. If the placental edge reaches (touches) the internal os the diagnosis is marginal placenta previa. Low-lying placenta is diagnosed when the distance between the internal cervical os and the placental edge is 1-20 mm.9, 10, 11, 12, 13, 14, 22, 23 If the distance between the internal cervical os and the placental edge is >20

Vasa previa

Vasa previa is a condition characterized by the presence of fetal blood vessels crossing or running in close proximity to the internal cervical os. Accurate prenatal diagnosis and timed preterm delivery are crucial to avoid premature rupture of membranes (PROM) that can lead to vessel tearing and rapid fetal bleeding. Risk factors for vasa previa include velamentous cord insertion, accessory placental lobe, twin or in vitro fertilization pregnancy, and marginal/low-lying placentations.7, 8

Velamentous cord insertion

Velamentous umbilical cord insertion is a condition characterized by insertion of umbilical cord vessels into the chorioamniotic membranes. Thus, the umbilical vessels are vulnerable to compression changes since they are not supported by Wharton jelly. This umbilical cord abnormality is seen in approximately 1% of pregnancies with incidences ranging from 0.48–2.4%.4, 46, 47, 48, 49 In general, preterm delivery is not recommended; however, this condition has been associated with increased

Summary and conclusions

Cesarean delivery is one of the most common of all surgical procedures, occurring in every third delivery in the United States.51 One of the consequences of increasing cesarean delivery rates over the last few decades is an increase in PIAs. This implies that we should not expect any reductions of preterm deliveries due to PIAs in the near future. Therefore, it is important to focus on strategies of how to improve the management of these patients. For these patients, the decisions regarding the

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    The authors report no conflict of interest.

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