text: 123-126 Placenta Previa
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  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: yinkamd@aol.com)
  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
  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%
  Copyright  2009 ISUOG. Published by John Wiley & Sons, Ltd.
  found that persistence was more likely in those placentae with a thin edge rather than a thick edge14 . Finally. Ghourab et al. This is because the lower uterine segment is only weakly contractile.2% of 1252 pregnancies had placenta previa in the first trimester. it makes sense that the risk of bleeding may be inversely proportional to the cervical length. found that only 6. was greater among those women with shorter cervices.21 retrospectively reviewed the charts of 86 patients who had a lowlying placenta.124
  Oyelese 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. In a recent study published in this Journal. Lauria et al.. especially if they have had some bleeding. the proportion of pregnancies thought to have placenta previa at 15–20 weeks was 1.11 and Becker et al. Bronsteen et al. In a study of 59 women with complete placenta previa. 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.
  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.
  . prior to about 34 weeks. severe bleeding in women
  Copyright  2009 ISUOG. Further studies are necessary in which the physicians are blinded to the ultrasound findings. published in the February 2009 edition of the White Journal. Ltd. and uterine contraction is the main mechanism for prevention of postpartum hemorrhage.
  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. Therefore. reduced this proportion several-fold. 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. to evaluate the likelihood of vaginal delivery in these women. Bhide et al. Smith et al. Saitoh et al. It should be emphasized. found that a distance of 2 cm using transvaginal sonography was usually associated with a successful attempt at vaginal delivery18 . There is the possibility that allowing an attempt at vaginal delivery when the placenta-to-os distance is ≥ 2 cm may be too conservative. The Cesarean section rate when this distance was less than 2 cm was over 90%. it was reported that approximately 20% of placentae covered the cervix or were low-lying at about 20 weeks8 . Published by John Wiley & Sons. when transabdominal sonography was used.1%. 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 . They found that while the risk of bleeding did not differ between women based on cervical length. A great confounder of all these studies is that the physicians were not blinded to the results of the ultrasound examinations. however. This suggests that the highest risk of sudden. usually due to bleeding. while 63% of women with a distance > 2 cm had vaginal deliveries. in a retrospective study of 121 cases of placenta previa. Transvaginal assessment of cervical length has been shown to be an effective tool in predicting preterm delivery. the risk of requiring an emergency Cesarean section at less than 34 weeks.3% rate among those women whose placenta was within 1 cm of the internal os.10 . the majority of these cases.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. Oppenheimer et al. 34: 123–126. due to its improved accuracy. In the first study specifically addressing this issue. Hill et al.
  Ultrasound Obstet Gynecol 2009. Dawson et al. They found a vaginal delivery rate of 76. with over 90% of women with placental edge-to-internal os distance < 2 cm having Cesarean deliveries. 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. performed weekly transvaginal sonograms from 28 weeks of gestation in 35 women with complete placenta previa17 . In fact.7 demonstrated that if transvaginal sonography was used. 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. defined as a placenta within 2 cm of the internal os within 4 weeks of delivery. are rarely severe or life-threatening and rarely require delivery. Prior to the widespread use of transvaginal sonography.
  of cases of placenta previa at 20 weeks resolve by term.. The risk of sudden. The challenge has been identifying who these women are. Similarly. Ghi et al.5% among patients who had a placenta-to-os distance of 1–2 cm and a 27. 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 . also found that 2 cm appeared to be the cut-off distance at which a vaginal delivery attempt was safe20 .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. their findings were almost identical to those of Dawson et al. In a later study. In 2002. It has been argued that women with placenta previa who are stable can be managed as outpatients15 . using transvaginal sonography. Transvaginal sonography. severe hemorrhage was 10 times as high in this group of patients compared with patients with other types of placenta previa. Since early bleeding in placenta previa may be due to cervical changes and uterine activity.16 evaluated transvaginal ultrasound cervical length in relation to risk of bleeding.
  Holmes P. 6. Accuracy and safety of transvaginal sonographic placental localization. Cervical length and risk of antepartum bleeding in women with complete placenta previa. Am J Obstet Gynecol 1988. Paul RH. 3. Holmes JH. 14. Ltd. The use of second-trimester transvaginal sonography to predict placenta previa. two conditions with which it is intimately associated. Delivery through an undiagnosed major placenta praevia: good outcome for mother and baby. 10. Perhaps the most important risk factor for placenta accreta is placenta previa in a woman who has had a prior Cesarean delivery. Martina T. Vasa previa is associated with a second-trimester lowlying placenta in at least two thirds of cases26 . Early identification of placenta praevia. Management of the symptomatic placenta previa: a randomized. Millar LK. 34: 123–126. equipment or personnel27 – 29 . Transvaginal ultrasonography for all placentas that appear to be low-lying or over the internal cervical os. Despite advances in technology. McClure N. Ultrasound Obstet Gynecol 1996. Hopefully. Wing DA. Varma TR. Lauria MR. 18: 96–99. It has been recognized that most cases of placenta accreta start off with an implantation in the anterior aspect of the lower uterus. Dornal JC. When the diagnosis is made prenatally. Ultrasound Obstet Gynecol 2001. Oyelese KO. and to determine who can attempt vaginal delivery. Ragosch V. Meriggiola MC. Lee W. Ville Y. Third-trimester transvaginal ultrasonography in placenta previa: does the shape of the lower placental edge predict clinical outcome? Ultrasound Obstet Gynecol 2001. Mende BC. women who have a second-trimester complete placenta previa should have a sonogram in the early third trimester to rule out vasa previa. This risk increases with the number of prior Cesarean sections23 . 18: 103–108. while vasa previa presents a tremendous risk to the fetus22 .
  Ultrasound has improved the diagnosis and management of placenta previa. Morandi R. J Obstet Gynaecol 1998. This management approach is associated with the lowest mortality and morbidity. Placenta previa–is the traditional diagnostic approach satisfactory? J Clin Ultrasound 1990. Hill LM. All patients with placenta previa should be screened for these two conditions. Comstock CH. 2. 4. Arnold MJ. to select patients who
  Copyright  2009 ISUOG. Treadwell MC. 7. avoiding the morbidity associated with Cesarean delivery. The implication of a low implantation of the placenta detected by ultrasonography in early pregnancy. Obstet Gynecol 1990. Bottoms SF. Oyelese Y. Ultrasonic placentography–a new method for placental localization. Am J Obstet Gynecol 1996. Smith RS. Dabrowski A. Bottoms SF. Leerentveld RA. Fox HE. while the perinatal mortality rate is at least 56% when the diagnosis is not made prenatally26 . controlled trial of inpatient versus outpatient expectant management. Vaginal ultrasound for diagnosis of placenta previa. Kirk JS. 5. 18: 385–386. Pilu G. almost 100% of babies survive. Manyonda IT. There is an increased risk even when a second-trimester lowlying placenta resolves in the third trimester26 . Thompson HE. Ultrasound Obstet Gynecol 2001. Becker RH. Both are associated with significant morbidity and mortality: placenta accreta is particularly risky to the mother. In both conditions. we will see several of these questions answered in the pages of this Journal. with the highest sensitivities and lowest false-positive rates. Evidence suggests that prenatal diagnosis makes all the difference to perinatal outcome in cases of vasa previa. 96: 538–547. Timor-Tritsch IE. A follow-up scan in the second trimester may help support the diagnosis. Gottesfeld KR. Gilberts EC. 175: 806–811. 9: 22–24. Placenta previa and vasa previa: time to leave the Dark Ages. the continuing challenge will be to use ultrasound to predict persistence to delivery. Kirk JS. Ultrasound Obstet Gynecol 1995. Turner M. Am J Obstet Gynecol 1966. Ikomi A.
  may be managed conservatively as outpatients. For most women with a prenatal sonographic diagnosis of placenta accreta. and both false-positive and fasle-negative diagnoses can be avoided. Farine D. especially early in the third trimester. 13. Contro E. Lauria MR. Timor-Tritsch IE. Ultrasound Obstet Gynecol 1997. Ultrasound Obstet Gynecol 2009. Smith RS. Acta Obstet Gynecol Scand 1981. Taylor ES.
  1. Comstock CH. Rizzo N. Ghourab S. in the years to come. 8. 97: 959–961. Transvaginal sonographic evaluation of first-trimester placenta previa. Morselli-Labate AM. Published by John Wiley & Sons. Diagnosis of low-lying placenta: can migration in the third trimester predict outcome? Ultrasound Obstet Gynecol 2001. 18: 100–102. Peisner DB. 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. and does not require increased cost. De Aloysio D. The relevance of placental location at 20–23 gestational weeks for prediction of placenta previa at delivery: evaluation of 8650 cases. 17: 496–501.Editorial
  Diagnosing placenta accreta and vasa previa
  No discussion of placenta previa would be complete without discussing placenta accreta and vasa previa. 11. suggesting that placenta accreta results from abnormal implantation in the deficient decidua or myometrium of the scar25 . 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 . prenatal diagnosis has a significant impact on outcome22 . Simpson N. Although transvaginal sonography now has widespread acceptance. Large studies have shown that routine screening for vasa previa at the time of the midtrimester scan is feasible and accurate. Farine D. 60: 265–268. 15. The lack of a retroplacental clear space is not a reliable sign and may occur in cases with placentae that are not abnormally adherent24 . timed Cesarean delivery followed by hysterectomy without attempted placental removal is the appropriate treatment. Jakobson S. Br J Obstet Gynaecol 1990. DiNofrio DM. 5: 301–303. Ghi T. Chenevey P. Wladimiroff JW. 16. 76: 759–762.
  . This makes the prospect of first-trimester screening for placenta accreta potentially feasible. Pelusi G. 159: 566–569. Vonk R. 18: 328–330. Virtually all cases can now be diagnosed sonographically. Orsini LF. Piva M. 9. Entezami M. Thus. Treadwell MC. 12. Lee W. Oppenheimer L. 33: 209–212.
  Ultrasound Obstet Gynecol 2009. 8: 337–340.
  Gynecol Obstet Invest 2002. Placental edge to internal os distance in the late third trimester and mode of delivery in placenta praevia. 15: 441–446. Balasubramaniam M. and clinical outcome.
  Oyelese
  24. Romano WM. Catanzarite V. Lewinsky RM. Gratton RJ. Koonings PP. Sloan CT. Dawson WB. Ultrasound Obstet Gynecol 2009. 18. Lee W. Kirk JS. 190: 1135–1140. Gagnon R. 21. Obstet Gynecol 2004. Araki T. placenta accreta. Rojas I. Bronsteen R. 54: 37–42. Tovbin Y. Blackwell S. Am J Obstet Gynecol 2004. quiz 24–26. Prefumo F.
  Copyright  2009 ISUOG. Published by John Wiley & Sons. Vasa previa: the impact of prenatal diagnosis on outcomes. BJOG 2003.
  Probe hygiene
  All the better to scan you with. Bronsteen RA. Ishihara K. Prenatal sonographic diagnosis of vasa previa: ultrasound findings and obstetric outcome in ten cases. 29. Obstet Gynecol 2006. Catanzarite V. Clark SL. Smulian JC. Comstock CH. Vasa previa: prenatal diagnosis. Lee W. Obstet Gynecol 1985. 27.interscience. Anticipation of uterine bleeding in placenta previa based on vaginal sonographic evaluation.. Huang RR. Oppenheimer LW. Comstock CH. Valice R. 19. Ultrasound Obstet Gynecol 2001. Fairbanks LA. Dumas MD.wiley. Goldstein V. Robert JA. Vettraino IM. Mowbray RD.
  Ultrasound Obstet Gynecol 2009. 23. Hollis B. Oyelese Y. 22. Sonographic detection of placenta accreta in the second and third trimesters of pregnancy. Vettraino IM. Thilaganathan B. 165: 1036–1038. Oyelese Y. 26. Lee W. Alcalde JL. Lashley S.
  Published online in Wiley InterScience (www.126
  17.1002/uog. Ltd. Prenatal detection of velamentous insertion of the umbilical cord: a prospective color Doppler ultrasound study. 18: 109–115. and vasa previa. 25. 22: 19–23. Mendoza A. natural evolution. 33: 204–208. Comstock C. Maida C. Bronsteen RA. 107: 927–941. Ritchie JW.com) DOI:10. 28. 110: 860–864. Moore J. Thomas W. Lorenz RP. Sepulveda W. Piacquadio KM. Sekiya T. Translabial ultrasonography and placenta previa: does measurement of the os-placenta distance predict outcome? J Ultrasound Med 1996. Obstet Gynecol 2000. What is a low-lying placenta? Am J Obstet Gynecol 1991. Phelan JP. Telford J. Ultrasound Obstet Gynecol 2003.
  . Placenta previa/accreta and prior cesarean section. Smulian JC. 103: 937–942. Love JJ Jr. Placenta previa. J Ultrasound Med 2003.7318. The early sonographic appearance of placenta accreta. Effect of a low-lying placenta on delivery outcome. Prefumo F. Stanco L. Saitoh M. 34: 123–126. Farine D. Bhide A. Schachter M. Lee VL. 21: 564–569. Lee W. Comstock CH. 95: 572–576. Schnapp C. 20. 66: 89–92. Smith RS.
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