Placental Location and Low-Lying Placenta: Measurement of Distance to Internal Cervical Os
Measurement Technique
The key parameter is the shortest distance from the lower (leading) edge of the placenta to the internal cervical os. This measurement determines the delivery approach in cases of low-lying placenta and placenta previa.
According to the source, endovaginal (transvaginal) assessment is preferred when placental location is uncertain or clinically significant for evaluation. This allows more accurate visualization of the relationship between the placental edge and the internal cervical os.
Threshold Values (ISUOG, 2024)
According to ISUOG Practice Guidelines (third trimester, 2024) for low-lying placenta:
| Distance from leading placental edge to internal cervical os | Management |
|---|---|
| ≥20 mm | Vaginal delivery is considered a safe option |
| 10–20 mm (at 36 weeks) | Safe vaginal delivery is also possible |
Clinical Context
Assessment of placental location is part of the third-trimester ultrasound examination protocol. When placenta previa is suspected and distance values are borderline, particular attention is paid to measurement accuracy—using transvaginal approach.
As part of placental assessment, signs of morbidly adherent placenta (PAS) are also considered. According to the source, the accuracy of PAS detection in contemporary series reaches approximately 90%; however, 5–10% of cases are identified only during cesarean delivery, with heterogeneity persisting in the described ultrasound findings.
Frequently asked questions
What distance from the placental edge to the cervical os is considered safe for vaginal delivery?
According to ISUOG (2024), at a distance of ≥20 mm from the leading placental edge to the internal cervical os, vaginal delivery is considered a safe option.
What should be done when the distance is 10–20 mm?
According to ISUOG (2024), at a distance between 10 and 20 mm at 36 weeks, safe vaginal delivery is also possible.
Which approach is preferred for assessing placental location?
Endovaginal (transvaginal) assessment is preferred when placental location is uncertain or clinically significant for evaluation.
How exactly is the distance to the cervical os measured?
The shortest distance is measured from the lower (leading) edge of the placenta to the internal cervical os.
How reliable is ultrasound in detecting morbidly adherent placenta (PAS)?
The accuracy of PAS detection in contemporary series is approximately 90%; however, 5–10% of cases are identified only during cesarean delivery.