Placenta Previa: Distance to Internal Os and Mode of Delivery
Placenta Previa: Distance to Internal Os and Mode of Delivery
Abstract & Commentary
By John C. Hobbins, MD, Professor and Chief of Obstetrics, University of Colorado Health Sciences Center, Denver, is Associate Editor for OB/GYN Clinical Alert.
Dr. Hobbins reports no financial relationship to this field of study.
Synopsis: The authors of this large Italian study showed that two-thirds of patients with placentas 10-20 mm from the cervix and one-third of those between 1-10 mm can safely deliver vaginally, and explored the concept that patients with placental edges within 2 cm of the endocervix should be defined as having placenta previa.
Source: Vergani P, et al. Placenta previa: Distance to internal os and mode of delivery. Am J Obstet Gynecol 2009;201:266.e1-5.
It is unclear where the idea originated that the definition of placenta previa would include placentas that are within 2 cm of the endocervix. However, this concept has been so heavily ingrained in today's practice that it is rare now for a patient to escape a planned cesarean section once this diagnosis is made late in pregnancy.
A group from Italy recently published a study that dealt with outcomes of pregnancies where the placental edges were within 2 cm of the endocervix. Vergani and colleagues evaluated 120 patients diagnosed to have placenta previa by vaginal ultrasound examinations, which were done within 28 days of delivery. Since the study involved 14,973 patients delivering over a 5-year period, the incidence of placenta previa was 8/1000.
The brave management protocol included attempting vaginal deliveries for those whose placentas were less than 2 cm from the endocervix and, obviously, sectioning all those at 37-39 weeks whose placentas were overlapping the cervix.
Forty-two patients had planned cesarean sections for overlapping placentas. After excluding those patients whose placentas had moved away from the cervix by more than 2 cm before delivery and those whose last ultrasound examinations were more than 28 days prior to delivery, 53 patients remained. They were all given a trial of labor.
The results were divided into 2 categories. Group 1 included 24 patients with placentas that were 1-10 mm from the cervix; group 2 had 29 patients whose placentas were between 10-20 mm away. The rates of cesarean section were 75% for group 1 vs 31% for group 2 (odds ratio [OR], 6.7; 95% confidence interval [CI], 2-26). The incidence of bleeding before labor was 29% vs 3%, respectively (OR, 11.5; 95% CI, 1.5-76). Blood loss during delivery and postpartum hemorrhage were essentially the same between groups.
Commentary
This study strongly suggests that 2 of 3 patients with low-lying placentas (10-20 mm from the cervix) can deliver safely by the vaginal route, and, surprisingly, even 1 of 3 highly motivated patients with placental edges that are within 10 mm from the cervix can deliver vaginally without major increased risk of intrapartum or postpartum hemorrhage.
There are clearly some limitations in the study such as bias in the management of these patients, since the providers were not blind to the ultrasound results. Another factor that may make the 10-20 mm group look better is that in some cases the last scans were performed up to 28 days prior to the scan in delivery — time enough for the placentas, through relative placental migration, to move into "non-previa" land.
Nevertheless, the authors of the study imply, Oppen-heimer and Farine in a companion editorial state,1 and I wholeheartedly agree, that the old definition of placenta previa, whose origin has been nebulous from the start, should be scrapped. Patients with ultrasound information within a week of delivery (and preferably performed on the labor and delivery floor on admission), indicating that their placentas are within 2 cm of the endocervix, now can be counseled with the above data in mind.
Reference
- Oppenheimer LW, Farine D. A new classification of placenta previa: Measuring progress in obstetrics. Am J Obstet Gynecol 2009;201;227-229.
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