Objective: To review the use of transvaginal ultrasound for the diagnosis of placenta previa and recommend management based on accurate placental localization.
Options: Transvaginal sonography (TVS) versus transabdominal sonography for the diagnosis of placenta previa; route of delivery,based on placenta edge to internal cervical os distance; in-patient versus out-patient antenatal care; cerclage to prevent bleeding; regional versus general anaesthesia; prenatal diagnosis of placenta accreta.
Outcome: Proven clinical benefit in the use of TVS for diagnosing and planning management of placenta previa.
Evidence: MEDLINE search for "placenta previa" and bibliographic review.
Benefits, harms, and costs: Accurate diagnosis of placenta previa may reduce hospital stays and unnecessary interventions.
Recommendations: 1. Transvaginal sonography, 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. (11-2A) 2. 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 os on TVS between 18 and 24 weeks' gestation (incidence 2-4%), 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) 3. When the placental edge lies between 20 mm away from the internal os and 20 mm of 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). (llI-B) 4. 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) 5. In general, any degree of overlap (> 0 mm) after 35 weeks is an indication for Caesarean section as the route of delivery. (ll-2A) 6. 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) 7. There is insufficient evidence to recommend the practice of cervical cerclage to reduce bleeding in placenta previa. (llI-D) 8. Regional anaesthesia may be employed for CS in the presence of placenta previa. (II-2B) 9. 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) VALIDATION: Comparison with Placenta previa and placenta previa accreta: diagnosis and management. Royal College ofObstetricians and Gynaecologists, Guideline No. 27,October 2005.The level of evidence and quality of recommendations are described using the criteria and classifications of the Canadian Task Force on Preventive Health Care (Table).