BACKGROUND AND PURPOSE:

• Most cases of placenta previa and low-lying placenta seen on second trimester ultrasound will no longer be present on the follow-up third trimester scan
• Jansen et al. (Ultrasound in Obstetrics & Gynecology, 2020) sought to assess the accuracy of the placental edge to internal os distance (IOD) measurement for the prediction of third trimester low-positioned placenta

METHODS:

• Prospective cohort study
• Participants
  • Pregnant women between 18 and 24 weeks
  • Undergoing TV ultrasound
  • Presence of low-positioned placenta
• Study design
  • Low-positioned placenta: IOD <20 mm
    • Placenta previa: Placenta covering the internal os of the cervix | Low-lying placenta: Placenta within 20 mm
  • All women also underwent follow-up third trimester ultrasound scan
  • Relative risk (RR) for a low-positioned placenta in the third trimester was calculated for the following  
    • Placenta previa vs a low-lying placenta
    • Posterior vs anterior placenta
    • Positive vs negative history of cesarean delivery
  • Multilevel likelihood ratios for ranges of IOD in the prediction of a low-positioned placenta in the third trimester were calculated separately for anteriorly and posteriorly located placentae

RESULTS:

• 958 women
• Placental location at second trimester
  • Placentae were more frequently posterior (62.0%) vs anterior (38.0%)
• 5.0% of women had low-positioned placenta that persisted into the third trimester

Persistence of Low-positioned Placenta in the Third Trimester

• Placenta previa vs low-lying placenta at time of second trimester scan
  • Placenta previa: 20.4%
  • Low-lying placenta: 1.4%
  • RR 17.9 (95% CI, 8.9 to 36.0)
• Posterior vs anterior placental location on second trimester scan 
  • Posterior: 6.4%
  • Anterior: 2.7%
  • RR 2.4 (95% CI, 1.2 to 4.9)
• Women with vs without a history of cesarean
  • With previous cesarean: 13.3%
  • Without: 4.0%
  • RR 3.7 (95% CI, 1.9 to 7.2)

Cut-off Value of IOD in the Second Trimester to Identify All Cases of an Abnormally Located Placenta in the Third Trimester

• For posterior placentae: 15.5 mm
• For anterior placentae: -4.5 mm (4.5-mm overlap of the placental edge over the internal os of the cervix)

CONCLUSION:

• A low-positioned placenta is more likely to persist into the third trimester if it is posteriorly located
• The authors state that the current IOD threshold of 20 mm could potentially be reduced

…in posteriorly located placentae, an IOD of 15.5 mm in the second trimester is associated with a negligible risk for placenta previa or low‐lying placenta in the third trimester

…for anteriorly located placentae, even one covering the internal os of the cervix by up to 4.5 mm in the second trimester is likely to resolve by the third trimester

Using these cut‐off values, the number of unnecessary follow‐up ultrasound examinations could be substantially decreased, notwithstanding the current advice to rescan all women with a low‐positioned placenta

Learn More – Primary Sources:

Follow‐up ultrasound in second‐trimester low‐positioned anterior and posterior placentae: prospective cohort study

Related ObG Topics:

Placenta Previa – Nomenclature, Diagnosis and Clinical Management

Vaginal Delivery or Cesarean Section for a Low-Lying Placenta?

Vasa Previa: Prevalence and Risk Factors

Is Placental Location Other than Previa Linked to Adverse Perinatal Outcomes?