Definitions of the levels of evidence (I, II-1, II-2, II-3, and III) and grades of recommendations (A-E and I) are provided at the end of the "Major Recommendations" field.
Diagnosis of Placenta Previa
- Transvaginal sonography (TVS), if available, may be used to investigate placental location at any time in pregnancy when the placenta is thought to be low-lying. It is significantly more accurate than transabdominal sonography, and its safety is well established. (ll-2A)
Prediction of Placenta Previa at Delivery
- Sonographers are encouraged to report the actual distance from the placental edge to the internal cervical os at TVS, using standard terminology of millimetres away from the os or millimetres of overlap. A placental edge exactly reaching the internal os is described as 0 mm. When the placental edge reaches or overlaps the internal cervical os on TVS between 18 and 24 weeks' gestation (incidence 2 to 4%), a follow-up examination for placental location in the third trimester is recommended. Overlap of more than 15 mm is associated with an increased likelihood of placenta previa at term. (ll-2A)
- When the placental edge lies between 20 mm away from the internal os and 20 mm overlap after 26 weeks' gestation, ultrasound should be repeated at regular intervals depending on the gestational age, distance from the internal os, and clinical features such as bleeding, because continued change in placental location is likely. Overlap of 20 mm or more at any time in the third trimester is highly predictive of the need for Caesarean section (CS). (lll-B)
Route of Delivery at Term
- The os–placental edge distance on TVS after 35 weeks' gestation is valuable in planning route of delivery. When the placental edge lies > 20 mm away from the internal cervical os, women can be offered a trial of labour with a high expectation of success. A distance of 20 to 0 mm away from the os is associated with a higher CS rate, although vaginal delivery is still possible depending on the clinical circumstances. (ll-2A)
- In general, any degree of overlap (> 0 mm) after 35 weeks is an indication for Caesarean section as the route of delivery.(ll-2A)
Inpatient Versus Outpatient Management
- Outpatient management of placenta previa may be appropriate for stable women with home support, close proximity to a hospital, and readily available transportation and telephone communication. (ll-2C)
Cervical Cerclage
- There is insufficient evidence to recommend the practice of cervical cerclage to reduce bleeding in placenta previa. (lll-D)
Method of Anaesthesia for Caesarean Section
- Regional anaesthesia may be employed for CS in the presence of placenta previa. (II-2B)
Placenta Previa and Placenta Accreta
- Women with a placenta previa and a prior CS are at high risk for placenta accreta. If there is imaging evidence of pathological adherence of the placenta, delivery should be planned in an appropriate setting with adequate resources. (II-2B)
Definitions:
Quality of Evidence Assessment*
I: Evidence obtained from at least one properly randomized controlled trial
II-1: Evidence from well-designed controlled trials without randomization
II-2: Evidence from well-designed cohort (prospective or retrospective) or case-control studies, preferably from more than one centre or research group
II-3: Evidence obtained from comparisons between times or places with or without the intervention. Dramatic results in uncontrolled experiments (such as the results of treatment with penicillin in the 1940s) could also be included in this category
III: Opinions of respected authorities, based on clinical experience, descriptive studies, or reports of expert committees
Classification of Recommendations**
A. There is good evidence to recommend the clinical preventive action.
B. There is fair evidence to recommend the clinical preventive action<./p>
C. The existing evidence is conflicting and does not allow to make a recommendation for or against use of the clinical preventive action; however, other factors may influence decision-making.
D. There is fair evidence to recommend against the clinical preventive action.
E. There is good evidence to recommend against the clinical preventive action.
I. There is insufficient evidence (in quantity or quality) to make a recommendation; however, other factors may influence decision-making.
*The quality of evidence reported in these guidelines has been adapted from the Evaluation of Evidence criteria described in the Canadian Task Force on the Periodic Preventive Health Exam Care.
**Recommendations included in these guidelines have been adapted from the Classification of Recommendations criteria described in the Canadian Task Force on the Periodic Preventive Health Exam Care.
