Dashe JS, McIntire DD, Ramus RM, Santos-Ramos R, Twickler DM
Obstetrics & Gynecology. 2002;99(5):692-697

 

Site: University of Texas, Southwestern Medical Center, Dallas

Objectives: To correlate the gestational age at ultrasound detection of placenta previa with the persistence of previa until delivery. To assess the effects of previa type, parity, and prior cesarean delivery on previa persistence.

Methods: This was a retrospective cohort study of 714 pregnancies with placenta previa detected during transabdominal or endovaginal ultrasound examination. Previa was categorized as "complete" if the placenta completely covered the internal cervical os. Gestational age was grouped into 4-week intervals from 15 to 36 weeks. The outcome was cesarean delivery for persistent previa.

Results: Of those with placenta previa at 15-19 weeks, 20-23 weeks, 24-27 weeks, 28-31 weeks, and 32-35 weeks, previa persisted until delivery in 12%, 34%, 49%, 62%, and 73%, respectively. At each interval, complete previa was more likely to persist than incomplete previa, all P < .001. Prior cesarean delivery was an independent risk factor for persistent previa among women diagnosed with previa in the second trimester, P < .05. However, parity was not an independent risk factor for persistence at any gestational age after adjusting for prior cesarean delivery.

Conclusions: Gestational age at ultrasound detection of placenta previa may be used to predict likelihood of previa persistence. After midpregnancy, risk of persistence seems to be higher than previously reported.

 

Commentary

This study reinforces the view that a high percentage of "placenta previas" diagnosed at second trimester ultrasound do "migrate" as pregnancy advances. The most important conclusion is that, whereas parity is a risk factor for previa prevalence at screening ultrasound, it does not seem to be a risk factor for previa persistence at time of delivery unless there was a prior cesarean section.

There are many flaws in this report, based largely on its retrospective nature and incomplete data. No information is presented about the total number of deliveries during the 10-year study period, nor about the percentage of patients who had "routine" ultrasound. How many "previas" were diagnosed by abdominal versus transvaginal ultrasound?

It is likely that those picked up between 15 and 24 weeks were incidental findings at abdominal screening sonograms. Only 940 ultrasounds were performed in the 714 patients: this implies that only 1 ultrasound was done in most of these pregnancies, and begs the question, How was the persistence of previa diagnosed? We know that 215 of these women (30%) underwent cesarean for "persistent previa," but there are no data on whether the patients presented with bleeding or were sectioned on the basis of a single earlier ultrasound. Except in cases of central previa, accreta, or percreta, it is not easy to tell at cesarean whether the placenta completely covers the cervical os.

It would have been interesting to read about the clinical course and management of these patients, since the diagnosis of "placenta previa" inspires anxiety in both patients and physicians. Yet, in 20 years of busy obstetric practice, this reviewer can recall only 1 case of previa requiring hysterectomy: she had undergone previous cesarean delivery and had a placenta accreta involving the lower segment behind the bladder. This low incidence is in keeping with data presented in 1994 by Zelop and colleagues^[1]: "placenta previa" was diagnosed at 2nd-trimester ultrasound in 925 patients; 267 had cesarean delivery, but in only 43 (4.6%) was the diagnosis of "previa" obvious at the time of surgery; of these, only 21 had antepartum bleeding.

An ultrasound report of "placenta previa" cannot be ignored, but neither should it prompt overreaction. In the asymptomatic patient, a transvaginal scan performed late in the 2nd trimester will help sort out the real previas from the "low-lying" placentas.

Two recent articles, from Canada^[2] and Germany,^[3] showed that placental "migration" can continue through the 3rd trimester. However, all patients where the placental edge overlapped the cervical os by more than 20 mm² or 25 mm³ required cesarean delivery.

When complete placenta previa persists in the 3rd trimester, it seems sensible to counsel the patient about avoiding intercourse or excessive activity and to go to the hospital promptly in the event of bleeding or contractions. The possibility of blood transfusions or hysterectomy should also be discussed.

Symptomatic placenta previa has traditionally been managed by hospitalization until delivery. Wing and coworkers^[4] showed that outpatient expectant management was safe in selected patients and much more cost-effective.

 

References

1. Zelop CC, Bromley B, Frigoletto FD Jr, Benacerraf BR. Second trimester sonographically diagnosed placenta previa: prediction of persistent previa at birth. Int J Gynaecol Obstet. 1994;44:207-210.
2. 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.
3. 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.
4. 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.