Placenta previa: the evolving role of ultrasound - Wiley Online Library

Ultrasound Obstet Gynecol 2009; 34: 123–126 Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/uog.7312

Editorial Placenta previa: the evolving role of ultrasound Y. OYELESE Tennessee Institute of Fetal Maternal and Infant Health, Department of Ob/Gyn, Division of Maternal Fetal Medicine, 853 Jefferson Ave, Suite E102, University of Tennessee Health Sciences Center, Memphis, TN 38163, USA (e-mail: [email protected])

Introduction My interest in placenta previa began when I observed, unintentionally, a woman deliver her baby vaginally through a complete placenta previa1 , which had been diagnosed incorrectly as being located normally by transabdominal sonography. While the patient had previously had a second-trimester transabdominal ultrasound examination that showed complete placenta previa, the followup transabdominal sonogram at about 32 weeks had failed to reveal it, possibly due to engagement of the head. Thus, she was managed in labor as a patient without placenta previa. Fortunately, the outcome was good for both mother and fetus. It should be noted that this event occurred before transvaginal sonography was being used widely in the diagnosis of placenta previa. As a result of the incident, I developed a keen interest in transvaginal sonography in the diagnosis and evaluation of placenta previa. In 1966, Gottesfeld et al.2 made a major contribution to the modern management of placenta previa when they described the use of transabdominal ultrasound in determining placental location and in diagnosing placenta previa. Since then, the prenatal diagnosis of placenta previa by ultrasound has become fairly commonplace. In fact, most cases are now diagnosed at the time of the routine second-trimester ultrasound examination rather than following bleeding in the late second or early third trimester, as was previously the case3 . The next major advance came in 1988 when Farine et al.4 described the use of transvaginal ultrasound in the diagnosis of placenta previa. Transvaginal sonography has been shown to be more accurate than transabdominal sonography5,6 and was shown, quite unexpectedly, to be safe and well-tolerated, not leading to any increase in vaginal bleeding. In fact, studies have demonstrated that transabdominal sonography is associated with incorrect diagnoses about 25% of the time, while transvaginal sonography is almost always accurate7 . Despite some initial resistance, the use of transvaginal sonography for the diagnosis of placenta previa now has widespread

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acceptance. Recent United States data have shown a decrease in the incidence of placenta previa that is particularly unexpected with the ever-increasing numbers of Cesarean deliveries; the only plausible explanation is the more accurate ascertainment of placenta previa resulting from the more liberal use of transvaginal sonography. Clearly, ultrasound plays a central role in the diagnosis and management of placenta previa and the overwhelming majority of cases are now diagnosed on routine sonography in the second trimester. So, what further role can ultrasound play in the management of placenta previa?

More accurate diagnosis of second-trimester placenta previa and better prediction of which cases will persist to term Several years ago, it was observed that most women with placenta previa diagnosed by second-trimester sonography no longer had placenta previa at term8,9 . This phenomenon, called trophotropism, is due to development of the lower uterine segment. It is estimated that over 90%

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of cases of placenta previa at 20 weeks resolve by term. Prior to the widespread use of transvaginal sonography, when transabdominal sonography was used, it was reported that approximately 20% of placentae covered the cervix or were low-lying at about 20 weeks8 . Transvaginal sonography, due to its improved accuracy, reduced this proportion several-fold. Hill et al.10 , using transvaginal sonography, found that only 6.2% of 1252 pregnancies had placenta previa in the first trimester. Smith et al.7 demonstrated that if transvaginal sonography was used, the proportion of pregnancies thought to have placenta previa at 15–20 weeks was 1.1%. These investigators also found that a placenta that was overlying the internal os by > 1 cm at 15–20 weeks was predictive of placenta previa at delivery with 100% sensitivity and 86% specificity11 . Lauria et al.11 and Becker et al.12 similarly found that it was only those women who had a placenta that actually overlapped the cervix at 15–24 weeks who persisted to delivery. It has also been suggested that the rate of placental migration may be helpful in determining which cases of placenta previa are likely to persist to term13 . Finally, Ghourab et al. found that persistence was more likely in those placentae with a thin edge rather than a thick edge14 .

Helping to identify which women with placenta previa require hospitalization due to risk of severe bleeding Despite the fact that two-thirds of women with placenta previa have some antepartum bleeding, the majority of these cases, prior to about 34 weeks, are rarely severe or life-threatening and rarely require delivery. It has been argued that women with placenta previa who are stable can be managed as outpatients15 . The challenge has been identifying who these women are, especially if they have had some bleeding. Transvaginal assessment of cervical length has been shown to be an effective tool in predicting preterm delivery. Since early bleeding in placenta previa may be due to cervical changes and uterine activity, it makes sense that the risk of bleeding may be inversely proportional to the cervical length. In a study of 59 women with complete placenta previa, published in the February 2009 edition of the White Journal, Ghi et al.16 evaluated transvaginal ultrasound cervical length in relation to risk of bleeding. They found that while the risk of bleeding did not differ between women based on cervical length, the risk of requiring an emergency Cesarean section at less than 34 weeks, usually due to bleeding, was greater among those women with shorter cervices. Similarly, Saitoh et al. performed weekly transvaginal sonograms from 28 weeks of gestation in 35 women with complete placenta previa17 . They found that the risk of sudden hemorrhage was much higher in those women with an echo-free space in the placental edge overlying the internal os. The risk of sudden, severe hemorrhage was 10 times as high in this group of patients compared with patients with other types of placenta previa. This suggests that the highest risk of sudden, severe bleeding in women


with placenta previa is actually in those women who have a placenta that just overlies the internal os and in whom this edge contains a large sinus or echo-free space.

Determining candidates for vaginal delivery An important question that is often asked is how far the placenta needs to be from the internal os in order to allow a safe vaginal delivery. In the first study specifically addressing this issue, Oppenheimer et al. found that a distance of 2 cm using transvaginal sonography was usually associated with a successful attempt at vaginal delivery18 . In a later study, Dawson et al.19 found that 63% of women with a placental edge-to-internal os distance of greater than 2 cm on translabial ultrasound who were allowed to labor had a vaginal delivery safely. The Cesarean section rate when this distance was less than 2 cm was over 90%. In 2002, Bhide et al., in a retrospective study of 121 cases of placenta previa, also found that 2 cm appeared to be the cut-off distance at which a vaginal delivery attempt was safe20 . In fact, their findings were almost identical to those of Dawson et al., with over 90% of women with placental edge-to-internal os distance < 2 cm having Cesarean deliveries, while 63% of women with a distance > 2 cm had vaginal deliveries. A great confounder of all these studies is that the physicians were not blinded to the results of the ultrasound examinations. Therefore, physicians who traditionally felt that 2 cm was the safe distance may have inherently been biased towards a lower threshold for Cesarean section when they knew that the distance between the placenta and the internal os was less than 2 cm. There is the possibility that allowing an attempt at vaginal delivery when the placenta-to-os distance is ≥ 2 cm may be too conservative. In a recent study published in this Journal, Bronsteen et al.21 retrospectively reviewed the charts of 86 patients who had a lowlying placenta, defined as a placenta within 2 cm of the internal os within 4 weeks of delivery. They found a vaginal delivery rate of 76.5% among patients who had a placenta-to-os distance of 1–2 cm and a 27.3% rate among those women whose placenta was within 1 cm of the internal os. These findings suggest that the current guidelines may be changed to allow women with a placental edge-to-os distance of 1–2 cm to attempt a vaginal delivery. Further studies are necessary in which the physicians are blinded to the ultrasound findings, to evaluate the likelihood of vaginal delivery in these women. It should be emphasized, however, that women with a placental edge within 4 cm of the internal os have an increased risk of postpartum hemorrhage regardless of mode of delivery. This is because the lower uterine segment is only weakly contractile, and uterine contraction is the main mechanism for prevention of postpartum hemorrhage.

Ultrasound Obstet Gynecol 2009; 34: 123–126.

Editorial

Diagnosing placenta accreta and vasa previa No discussion of placenta previa would be complete without discussing placenta accreta and vasa previa, two conditions with which it is intimately associated. Both are associated with significant morbidity and mortality: placenta accreta is particularly risky to the mother, while vasa previa presents a tremendous risk to the fetus22 . All patients with placenta previa should be screened for these two conditions, especially early in the third trimester. In both conditions, prenatal diagnosis has a significant impact on outcome22 . Perhaps the most important risk factor for placenta accreta is placenta previa in a woman who has had a prior Cesarean delivery. This risk increases with the number of prior Cesarean sections23 . The most important sonographic finding for predicting placenta accreta is the presence of large echolucent lacunae in the placenta in the region overlying the scar24 . The lack of a retroplacental clear space is not a reliable sign and may occur in cases with placentae that are not abnormally adherent24 . It has been recognized that most cases of placenta accreta start off with an implantation in the anterior aspect of the lower uterus, suggesting that placenta accreta results from abnormal implantation in the deficient decidua or myometrium of the scar25 . This makes the prospect of first-trimester screening for placenta accreta potentially feasible. A follow-up scan in the second trimester may help support the diagnosis. For most women with a prenatal sonographic diagnosis of placenta accreta, timed Cesarean delivery followed by hysterectomy without attempted placental removal is the appropriate treatment. This management approach is associated with the lowest mortality and morbidity. Vasa previa is associated with a second-trimester lowlying placenta in at least two thirds of cases26 . There is an increased risk even when a second-trimester lowlying placenta resolves in the third trimester26 . Thus, women who have a second-trimester complete placenta previa should have a sonogram in the early third trimester to rule out vasa previa. Evidence suggests that prenatal diagnosis makes all the difference to perinatal outcome in cases of vasa previa. When the diagnosis is made prenatally, almost 100% of babies survive, while the perinatal mortality rate is at least 56% when the diagnosis is not made prenatally26 . Large studies have shown that routine screening for vasa previa at the time of the midtrimester scan is feasible and accurate, and does not require increased cost, equipment or personnel27 – 29 .

Conclusion Ultrasound has improved the diagnosis and management of placenta previa. Virtually all cases can now be diagnosed sonographically, and both false-positive and fasle-negative diagnoses can be avoided. Although transvaginal sonography now has widespread acceptance, the continuing challenge will be to use ultrasound to predict persistence to delivery, to select patients who


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may be managed conservatively as outpatients, and to determine who can attempt vaginal delivery, avoiding the morbidity associated with Cesarean delivery. Despite advances in technology, further research is needed to improve the accuracy of screening for placenta accreta and vasa previa and to determine the findings that are most predictive of these conditions, with the highest sensitivities and lowest false-positive rates. Hopefully, in the years to come, we will see several of these questions answered in the pages of this Journal.

REFERENCES 1. Oyelese KO, Turner M, Ikomi A, Ville Y, Manyonda IT. Delivery through an undiagnosed major placenta praevia: good outcome for mother and baby. J Obstet Gynaecol 1998; 18: 385–386. 2. Gottesfeld KR, Thompson HE, Holmes JH, Taylor ES. Ultrasonic placentography–a new method for placental localization. Am J Obstet Gynecol 1966; 96: 538–547. 3. Oyelese Y. Placenta previa and vasa previa: time to leave the Dark Ages. Ultrasound Obstet Gynecol 2001; 18: 96–99. 4. Farine D, Fox HE, Jakobson S, Timor-Tritsch IE. Vaginal ultrasound for diagnosis of placenta previa. Am J Obstet Gynecol 1988; 159: 566–569. 5. Farine D, Peisner DB, Timor-Tritsch IE. Placenta previa–is the traditional diagnostic approach satisfactory? J Clin Ultrasound 1990; 18: 328–330. 6. Leerentveld RA, Gilberts EC, Arnold MJ, Wladimiroff JW. Accuracy and safety of transvaginal sonographic placental localization. Obstet Gynecol 1990; 76: 759–762. 7. Smith RS, Lauria MR, Comstock CH, Treadwell MC, Kirk JS, Lee W, Bottoms SF. Transvaginal ultrasonography for all placentas that appear to be low-lying or over the internal cervical os. Ultrasound Obstet Gynecol 1997; 9: 22–24. 8. McClure N, Dornal JC. Early identification of placenta praevia. Br J Obstet Gynaecol 1990; 97: 959–961. 9. Varma TR. The implication of a low implantation of the placenta detected by ultrasonography in early pregnancy. Acta Obstet Gynecol Scand 1981; 60: 265–268. 10. Hill LM, DiNofrio DM, Chenevey P. Transvaginal sonographic evaluation of first-trimester placenta previa. Ultrasound Obstet Gynecol 1995; 5: 301–303. 11. Lauria MR, Smith RS, Treadwell MC, Comstock CH, Kirk JS, Lee W, Bottoms SF. The use of second-trimester transvaginal sonography to predict placenta previa. Ultrasound Obstet Gynecol 1996; 8: 337–340. 12. Becker RH, Vonk R, Mende BC, Ragosch V, Entezami M. The relevance of placental location at 20–23 gestational weeks for prediction of placenta previa at delivery: evaluation of 8650 cases. Ultrasound Obstet Gynecol 2001; 17: 496–501. 13. 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. 14. Ghourab S. Third-trimester transvaginal ultrasonography in placenta previa: does the shape of the lower placental edge predict clinical outcome? Ultrasound Obstet Gynecol 2001; 18: 103–108. 15. Wing DA, Paul RH, Millar LK. Management of the symptomatic placenta previa: a randomized, controlled trial of inpatient versus outpatient expectant management. Am J Obstet Gynecol 1996; 175: 806–811. 16. Ghi T, Contro E, Martina T, Piva M, Morandi R, Orsini LF, Meriggiola MC, Pilu G, Morselli-Labate AM, De Aloysio D, Rizzo N, Pelusi G. Cervical length and risk of antepartum bleeding in women with complete placenta previa. Ultrasound Obstet Gynecol 2009; 33: 209–212.


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126 17. Saitoh M, Ishihara K, Sekiya T, Araki T. Anticipation of uterine bleeding in placenta previa based on vaginal sonographic evaluation. Gynecol Obstet Invest 2002; 54: 37–42. 18. Oppenheimer LW, Farine D, Ritchie JW, Lewinsky RM, Telford J, Fairbanks LA. What is a low-lying placenta? Am J Obstet Gynecol 1991; 165: 1036–1038. 19. Dawson WB, Dumas MD, Romano WM, Gagnon R, Gratton RJ, Mowbray RD. Translabial ultrasonography and placenta previa: does measurement of the os-placenta distance predict outcome? J Ultrasound Med 1996; 15: 441–446. 20. Bhide A, Prefumo F, Moore J, Hollis B, Thilaganathan B. Placental edge to internal os distance in the late third trimester and mode of delivery in placenta praevia. BJOG 2003; 110: 860–864. 21. Bronsteen R, Valice R, Lee W, Blackwell S, Balasubramaniam M, Comstock C. Effect of a low-lying placenta on delivery outcome. Ultrasound Obstet Gynecol 2009; 33: 204–208. 22. Oyelese Y, Smulian JC. Placenta previa, placenta accreta, and vasa previa. Obstet Gynecol 2006; 107: 927–941. 23. Clark SL, Koonings PP, Phelan JP. Placenta previa/accreta and prior cesarean section. Obstet Gynecol 1985; 66: 89–92.

24. Comstock CH, Love JJ Jr., Bronsteen RA, Lee W, Vettraino IM, Huang RR, Lorenz RP. Sonographic detection of placenta accreta in the second and third trimesters of pregnancy. Am J Obstet Gynecol 2004; 190: 1135–1140. 25. Comstock CH, Lee W, Vettraino IM, Bronsteen RA. The early sonographic appearance of placenta accreta. J Ultrasound Med 2003; 22: 19–23; quiz 24–26. 26. Oyelese Y, Catanzarite V, Prefumo F, Lashley S, Schachter M, Tovbin Y, Goldstein V, Smulian JC. Vasa previa: the impact of prenatal diagnosis on outcomes. Obstet Gynecol 2004; 103: 937–942. 27. Lee W, Lee VL, Kirk JS, Sloan CT, Smith RS, Comstock CH. Vasa previa: prenatal diagnosis, natural evolution, and clinical outcome. Obstet Gynecol 2000; 95: 572–576. 28. Catanzarite V, Maida C, Thomas W, Mendoza A, Stanco L, Piacquadio KM. Prenatal sonographic diagnosis of vasa previa: ultrasound findings and obstetric outcome in ten cases. Ultrasound Obstet Gynecol 2001; 18: 109–115. 29. Sepulveda W, Rojas I, Robert JA, Schnapp C, Alcalde JL. Prenatal detection of velamentous insertion of the umbilical cord: a prospective color Doppler ultrasound study. Ultrasound Obstet Gynecol 2003; 21: 564–569.

Probe hygiene

All the better to scan you with.

Published online in Wiley InterScience (www.interscience.wiley.com) DOI:10.1002/uog.7318.

