123-126 Placenta Previa

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Ultrasound Obstet Gynecol2009; 34: 123126Published 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 FetalMedicine, 853 Jefferson Ave, Suite E102, University ofTennessee 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 vaginallythrough a complete placenta previa1, which had beendiagnosed incorrectly as being located normally by trans-abdominal sonography. While the patient had previouslyhad a second-trimester transabdominal ultrasound exam-ination that showed complete placenta previa, the follow-up transabdominal sonogram at about 32 weeks hadfailed to reveal it, possibly due to engagement of thehead. Thus, she was managed in labor as a patient with-out placenta previa. Fortunately, the outcome was goodfor both mother and fetus. It should be noted that this

event occurred before transvaginal sonography was beingused widely in the diagnosis of placenta previa. As a resultof the incident, I developed a keen interest in transvaginalsonography in the diagnosis and evaluation of placentaprevia.

In 1966, Gottesfeld et al.2 made a major contributionto the modern management of placenta previa whenthey described the use of transabdominal ultrasound indetermining placental location and in diagnosing placentaprevia. Since then, the prenatal diagnosis of placentaprevia by ultrasound has become fairly commonplace. Infact, most cases are now diagnosed at the time of the

routine second-trimester ultrasound examination ratherthan following bleeding in the late second or early thirdtrimester, as was previously the case3.

The next major advance came in 1988 when Farineet al.4 described the use of transvaginal ultrasound in thediagnosis of placenta previa. Transvaginal sonographyhas been shown to be more accurate than transabdominalsonography5,6 and was shown, quite unexpectedly, to besafe and well-tolerated, not leading to any increase invaginal bleeding. In fact, studies have demonstrated thattransabdominal sonography is associated with incorrectdiagnoses about 25% of the time, while transvaginalsonography is almost always accurate7. Despite someinitial resistance, the use of transvaginal sonography forthe diagnosis of placenta previa now has widespread

acceptance. Recent United States data have shown adecrease in the incidence of placenta previa that isparticularly unexpected with the ever-increasing numbersof Cesarean deliveries; the only plausible explanationis the more accurate ascertainment of placenta previaresulting from the more liberal use of transvaginalsonography.

Clearly, ultrasound plays a central role in the

diagnosis and management of placenta previa and theoverwhelming majority of cases are now diagnosed onroutine sonography in the second trimester. So, whatfurther role can ultrasound play in the management ofplacenta previa?

More accurate diagnosis of second-trimester placentaprevia and better prediction of which cases will persistto term

Several years ago, it was observed that most womenwith placenta previa diagnosed by second-trimestersonography no longer had placenta previa at term8,9. Thisphenomenon, called trophotropism, is due to developmentof 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 wasreported that approximately 20%of placentae covered thecervix or were low-lying at about 20 weeks8. Transvaginalsonography, due to its improved accuracy, reduced thisproportion several-fold. Hill et al.10, using transvaginal

sonography, found that only 6.2% of 1252 pregnancieshad placenta previa in the first trimester. Smith et al.7

demonstrated that if transvaginal sonography was used,the proportion of pregnancies thought to have placentaprevia at 15 20 weeks was 1.1%. These investigators alsofound that a placenta that was overlying the internal os by>1 cm at 15 20 weeks was predictive of placenta previaat delivery with 100% sensitivity and 86% specificity11.Lauria et al.11 and Becker et al.12 similarly found that itwas only those women who had a placenta that actuallyoverlapped the cervix at 15 24 weeks who persistedto delivery. It has also been suggested that the rate of

placental migration may be helpful in determining whichcases of placenta previa are likely to persist to term13.Finally, Ghourab et al. found that persistence was morelikely in those placentae with a thin edge rather than athick edge14.

Helping to identify which women with placenta previarequire hospitalization due to risk of severe bleeding

Despite the fact that two-thirds of women with placentaprevia have some antepartum bleeding, the majority ofthese cases, prior to about 34 weeks, are rarely severe orlife-threatening and rarely require delivery. It has been

argued that women with placenta previa who are stablecan be managed as outpatients15. The challenge has beenidentifying who these women are, especially if they havehad some bleeding. Transvaginal assessment of cervicallength has been shown to be an effective tool in predictingpreterm delivery. Since early bleeding in placenta previamay be due to cervical changes and uterine activity, itmakes sense that the risk of bleeding may be inverselyproportional 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 foundthat while the risk of bleeding did not differ betweenwomen based on cervical length, the risk of requiring anemergency Cesarean section at less than 34 weeks, usuallydue to bleeding, was greater among those women withshorter cervices.

Similarly, Saitoh et al. performed weekly transvaginalsonograms from 28 weeks of gestation in 35 women withcomplete placenta previa17. They found that the risk ofsudden hemorrhage was much higher in those womenwith an echo-free space in the placental edge overlyingthe internal os. The risk of sudden, severe hemorrhage was10 times as high in this group of patients compared withpatients with other types of placenta previa. This suggeststhat the highest risk of sudden, severe bleeding in women

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

Determining candidates for vaginal delivery

An important question that is often asked is howfar the placenta needs to be from the internal os inorder to allow a safe vaginal delivery. In the firststudy specifically addressing this issue, Oppenheimeret al. found that a distance of 2 cm using transvaginalsonography was usually associated with a successfulattempt at vaginal delivery18. In a later study, Dawsonet al.19 found that 63% of women with a placentaledge-to-internal os distance of greater than 2 cm ontranslabial ultrasound who were allowed to labor hada vaginal delivery safely. The Cesarean section ratewhen this distance was less than 2 cm was over 90%.In 2002, Bhide et al., in a retrospective study of121 cases of placenta previa, also found that 2 cmappeared to be the cut-off distance at which a vaginaldelivery attempt was safe20. In fact, their findingswere almost identical to those of Dawson et al., withover 90% of women with placental edge-to-internalos distance 2 cm had vaginaldeliveries. A great confounder of all these studies isthat the physicians were not blinded to the results ofthe ultrasound examinations. Therefore, physicians whotraditionally felt that 2 cm was the safe distance mayhave inherently been biased towards a lower threshold

for Cesarean section when they knew that the distancebetween the placenta and the internal os was less than2 cm.

There is the possibility that allowing an attempt atvaginal delivery when the placenta-to-os distance is2 cm may be too conservative. In a recent studypublished in this Journal, Bronsteenet al.21 retrospectivelyreviewed the charts of 86 patients who had a low-lying placenta, defined as a placenta within 2 cmof the internal os within 4 weeks of delivery. Theyfound a vaginal delivery rate of 76.5% among patientswho had a placenta-to-os distance of 1 2 cm and

a 27.3% rate among those women whose placentawas within 1 cm of the internal os. These findingssuggest that the current guidelines may be changed toallow women with a placental edge-to-os distance of1 2 cm to attempt a vaginal delivery. Further studiesare necessary in which the physicians are blindedto the ultrasound findings, to evaluate the likelihoodof vaginal delivery in these women. It should beemphasized, however, that women with a placentaledge within 4 cm of the internal os have an increasedrisk of postpartum hemorrhage regardless of mode ofdelivery. This is because the lower uterine segmentis only weakly contractile, and uterine contraction isthe main mechanism for prevention of postpartumhemorrhage.

Copyright 2009 ISUOG. Published by John Wiley & Sons, Ltd. Ultrasound Obstet Gynecol2009; 34: 123126.


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Diagnosing placenta accreta and vasa previa

No discussion of placenta previa would be completewithout discussing placenta accreta and vasa previa, twoconditions with which it is intimately associated. Bothare associated with significant morbidity and mortality:placenta accreta is particularly risky to the mother, while

vasa previa presents a tremendous risk to the fetus22

. Allpatients with placenta previa should be screened for thesetwo conditions, especially early in the third trimester.In both conditions, prenatal diagnosis has a significantimpact on outcome22.

Perhaps the most important risk factor for placentaaccreta is placenta previa in a woman who has hada prior Cesarean delivery. This risk increases with thenumber of prior Cesarean sections23. The most importantsonographic finding for predicting placenta accreta is thepresence of large echolucent lacunae in the placenta in theregion overlying the scar24. The lack of a retroplacentalclear space is not a reliable sign and may occur in cases

with placentae that are not abnormally adherent24. Ithas been recognized that most cases of placenta accretastart off with an implantation in the anterior aspect ofthe lower uterus, suggesting that placenta accreta resultsfrom abnormal implantation in the deficient decidua ormyometrium of the scar25. This makes the prospect offirst-trimester screening for placenta accreta potentiallyfeasible. A follow-up scan in the second trimester mayhelp support the diagnosis. For most women with aprenatal sonographic diagnosis of placenta accreta, timedCesarean delivery followed by hysterectomy withoutattempted placental removal is the appropriate treatment.

This management approach is associated with the lowestmortality and morbidity.

Vasa previa is associated with a second-trimester low-lying placenta in at least two thirds of cases26. Thereis an increased risk even when a second-trimester low-lying placenta resolves in the third trimester26. Thus,women who have a second-trimester complete placentaprevia should have a sonogram in the early third trimesterto rule out vasa previa. Evidence suggests that prenataldiagnosis makes all the difference to perinatal outcomein cases of vasa previa. When the diagnosis is madeprenatally, almost 100% of babies survive, while theperinatal mortality rate is at least 56% when the diagnosisis not made prenatally26. Large studies have shown thatroutine screening for vasa previa at the time of the mid-trimester scan is feasible and accurate, and does notrequire increased cost, equipment or personnel2729.

Conclusion

Ultrasound has improved the diagnosis and managementof placenta previa. Virtually all cases can now bediagnosed sonographically, and both false-positive andfasle-negative diagnoses can be avoided. Althoughtransvaginal sonography now has widespread acceptance,the continuing challenge will be to use ultrasound topredict persistence to delivery, to select patients who

may be managed conservatively as outpatients, and todetermine who can attempt vaginal delivery, avoidingthe morbidity associated with Cesarean delivery. Despiteadvances in technology, further research is needed toimprove the accuracy of screening for placenta accreta andvasa previa and to determine the findings that are mostpredictive of these conditions, with the highest sensitivities

and lowest false-positive rates. Hopefully, in the years tocome, we will see several of these questions answered inthe pages of this Journal.

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Probe hygiene

All the better to scan you with.

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


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