Role of ultrasound in placental evaluation
The placenta may be visualized as early as 6 weeks by transvaginal sonography and 10 weeks transabdominally. It first appears as a focally thickened hyperechogenic rim of tissue around the gestational sac that is clearly separate from the myometrium. It is quite distinct and easily identifiable at 14 to 15 weeks and the intervillous blood flow may be documented using color Doppler.3,11
The relationship between the placental edge and the internal cervical os changes with advancing gestation. As such, it is very important not to label the patient as having a placenta previa prior to 16 weeks.12 The placenta “migrates” away from the internal os as the lower uterine segment develops and a rate of migration of 5.4 mm per week has been reported.13 Furthermore over 98.4% of suspected low lying/placenta previas in the second trimester resolve prior to delivery, at a mean gestational age of 26 weeks, with only 1.6% persisting at term.14 Though a prior cesarean section6 and higher parity are risk factors for having a placenta previa, parity does not impact the rate of persistence.15
Ultrasound plays a critical role in placental localization whether at the point-of-care setting or during screening examinations. Some of the earliest reports from 1977 suggested that a low lying or placenta previa in early pregnancy may be a normal variant.16 Though this was initially described transabdominally, the availability of transvaginal scanning since the 1980s has allowed for more accurate diagnoses.5,17
Several techniques can be employed to evaluate the placenta, the lower uterine segment, and cord insertion to screen for a placenta previa. It is important to note that presence of a full maternal bladder or uterine contractions may lead to false diagnosis of a placenta previa (Figures 1-4, Table 2).
A simple sonographic technique for placental localization has been described as part of a standardized 6-Step approach for performance of the focused basic obstetric ultrasound examination.18 This technique is described with the transducer held in a sagittal orientation just above the uterine fundus and moved to the lower abdomen in three sweeps (maternal right, left and center). Subsequently, the position of the lower placental edge with respect to the cervix is assessed. If it is less than 2 cm or the placenta is felt to cover the cervix then confirmation by transvaginal scan is indicated.5
In case of persistence of a placenta previa into the third trimester, it is critical to assess for possible presence of placenta accreta spectrum. Presence of placental lacunae, loss of the clear retroplacental echolucent space, presence of hypervascularity and bridging vessels, thinning of the myometrium, thinning of the retroplacental myometrial wall and the placental bladder interphase, and a “bulging” placenta into the posterior bladder wall are all concerning findings for a placental accreta spectrum.8
It must be kept in mind that if there were to be normalization of a low-lying placenta or a placenta previa with advancing gestation, it is important to rule out an ensuing vasa previa, which is associated with increased fetal mortality if undiagnosed prenatally. It has been estimated that approximately 28% of prenatally diagnosed pregnancies with vasa previas require an emergent preterm delivery.19 Presence of echolucent or circular lines overlying the internal os on transabdominal or transvaginal ultrasound should alert the examiner to the presence of vasa previa. This can be confirmed by transvaginal assessment with color and spectral Doppler, confirming presence of arterial fetal vessels.20,21 Nonetheless, approximately 39% of vasa previas resolve in the third trimester.22
Timing of the examination is key to arriving at the correct diagnosis. The rate of persistence of a placenta previa is directly related to the gestational age at sonographic diagnosis. It has been determined to persist in 12% of those diagnosed at 15-19 weeks; 34% at 20-23 weeks; 39% at 24-27 weeks; 62% at 28-31 weeks and 73% at 32-35 weeks.15 In addition, it is important to keep in mind several tips and tricks when evaluating for a low lying/previa as summarized in Table 2.
The author reports no potential conflicts of interest with regard to this article.
- AIUM–ACR–ACOG–SMFM–SRU practice parameter for the performance of standard diagnostic obstetric ultrasound examinations. J Ultrasound Med. 2018;37:E13-E24.
- Salomon LJ, Alfirevic Z, Berghella V, Bilardo C, Hernandez‐Andrade E, Johnsen SL, et al. Practice guidelines for performance of the routine mid‐trimester fetal ultrasound scan. Ultrasound Obstet Gynecol. 2011;37:116-126.
- Kanne JP, Lalani TA, Fligner CL. The placenta revisited:radiologic–pathologic correlation. Probl Diagn Radiol. 2005;34(6):238-255.
- Taipale P, Hiilesmaa V, Ylöstalo P. Transvaginal ultrasonography at 18–23 weeks in predicting placenta previa at delivery. Ultrasound Obstet Gynecol. 1998;12:422-425.
- Jauniaux ERM, Alfirevic Z, Bhide AG, Belfort MA, Burton GJ, Collins SL, et al.. Placenta praevia and placenta accreta: diagnosis and management. Green-top Guideline No. 27a. BJOG. 2018.
- Ananth CV, Smulian JC, Vintzileos AM. The association of placenta previa with history of cesarean delivery and abortion: a meta-analysis. Am J Obstet Gynecol, 1997;177:1071-1078.
- Downes KL, Hinkle SN, Sjaarda LA, et al. Previous prelabor or intrapartum cesarean delivery and risk of placenta previa. Am J Obstet Gynecol 2015;212:669.e1-6.
- Oyelese Y, Smulian JC. Placenta previa, placenta accreta, and vasa previa. Obstet Gynecol. 2006;107(4):927-941.
- Balayla J, Desilets J, Shrem G. Placenta previa and the risk of intrauterine growth restriction (IUGR): a systematic review and meta-analysis. J Perinat Med. 2019;47(6): 577-584.
- Reddy UM, Abuhamad AZ, Levine D, Saade GR; Fetal Imaging Workshop Invited Participants. Fetal imaging: Executive summary of a joint Eunice Kennedy Shriver National Institute of Child Health and Human Development, Society for Maternal-Fetal Medicine, American Institute of Ultrasound in Medicine, American College of Obstetricians and Gynecologists, American College of Radiology, Society for Pediatric Radiology, and Society of Radiologists in Ultrasound Fetal Imaging Workshop. J Ultrasound Med. 2014;33:745-757.
- Fadl S, Moshiri M, Fligner CL, Katz DS Dighe M. Placental imaging: normal appearance with review of pathologic findings. RadioGraphics. 2017;37(3):979-998.
- Gallagher P, Fagan CJ, Bedi DG, Winsett MZ, Reyes RN. Potential placenta previa: Definition, frequency, and significance. Am J Roentgenol. 1987;149:1013-1015.
- Oppenheimer L, Holmes P, Simpson N, Dabrowski A. Diagnosis of low-lying placenta: can migration in the third trimester predict outcome? Ultrasound Obstet Gynecol. 2001;18:100-102.
- Heller HT, Mullen KM, Gordon RW, Reiss RE, Benson CB. Outcomes of pregnancies with a low‐lying placenta diagnosed on second‐trimester sonography. J Ultrasound Med. 2014;33: 691-696.
- Dashe JS, McIntire DD, Ramus RM, Santos-Ramos R, Twickler DM. Persistence of Placenta Previa According to Gestational Age at Ultrasound DetectionObstetrics & Gynecology: May 2002 - Volume 99 - Issue 5 - p 692–697.
- Wexler P, Gottesfeld KR. Second trimester placenta previa: an apparently normal placentation. Obstet Gynecol. 1977:50:706-709.
- Farine D, Fox HE, Jakobson S, Timor-Tritsch IE. Vaginal ultrasound for diagnosis of placenta previa. Am J Obstet Gynecol. 1988;159(3):566-569.
- Abuhamad AZ, Zhao Y, Abuhamad S, Sinkovskaya E, Rao R, Kanaan C, et al. Standardized six-step approach to the performance of the focused basic obstetric ultrasound examination. Am J Perinatol. 2016;33(1):90-98.
- Society of Maternal-Fetal (SMFM) Publications Committee: Sinkey RG, Odibo AO, Dashe JS.#37 Diagnosis and Management of Vasa Previa. AJOG 2015;213(5):615-619.
- Bhide A, Thilaganathan B. Recent advances in the management of placenta previa. Curr Opin Obstet Gynecol. 2004;16:447-451.
- Lee W, Lee V, Kirk JS, Sloan C, Smith R, Comstock C. Vasa previa: prenatal diagnosis, natural evolution, and clinical outcome. Obstet Gynecol. 2000;95(4):572-576.
- Klahr R, Fox NS, Zafman K, Hill MB, Connolly CT, Rebarber A. Frequency of spontaneous resolution of vasa previa with advancing gestational age. AJOG. 2019 June 21 (ePub ahead of print).
- Lockwood CJ, Russo-Stieglitz K. Placenta previa management. UpToDate. July 23, 2019.
- Spong CY, Mercer BM, D’Alton M, Kilpatrick S, Blackwell S, Saade G. Timing of indicated late-preterm and early-term birth. Obstet Gynecol. 2011;118(2 Part 1):323-333.
- American College of Obstetricians and Gynecologists. Medically indicated late-preterm and early-term deliveries. ACOG Committee opinion no. 560. Obstet Gynecol. 2013;121:908-910.
- Gyamfi-Bannerman C. Society for Maternal-Fetal Medicine (SMFM) Consult Series #44: Management of bleeding in the late preterm period. SLOG 2018;218(1):B2-B8.
- Oyelese Y, Society for Maternal-Fetal Medicine. MFM consult: Evaluation and management of low-lying placenta or placenta previa on second trimester ultrasound. Contemp Ob/Gyn. December 2010:30-3.
- Robinson B, Grobman W. Effectiveness of timing strategies for delivery of individuals with placenta previa and accreta. Obstet Gynecol. 2010; 116(4):835-842.