Low-Lying Anterior Placenta: Definition, Diagnosis, and Management

Diagnosis and Classification

A low-lying anterior placenta is diagnosed when the placental edge is within 2 cm of the internal cervical os but not covering it, while placenta previa occurs when the placenta completely overlies the internal os, as defined by the American College of Radiology 1, 2
Transvaginal ultrasound is the gold standard for accurate diagnosis and assessment of the distance between the placental edge and internal cervical os (IOD), according to the American College of Radiology 1, 2
Digital pelvic examination should be avoided until placenta previa has been excluded to prevent triggering hemorrhage, as recommended by the American College of Radiology 2
Transabdominal ultrasound should be performed initially, followed by transvaginal ultrasound for more precise measurement, as suggested by the American College of Radiology 1, 3
Doppler ultrasound is essential to identify any vessels overlying the internal cervical os and assess placental blood flow patterns, as recommended by the American College of Radiology 1, 3

Follow-up ultrasound in the third trimester (28-32 weeks) is essential to reassess placental position, as recommended by the American College of Radiology 4, 2
If the placenta is ≥2 cm from the internal cervical os at 18-20 weeks, follow-up evaluation is not necessary, according to the American College of Radiology 1
Delivery planning should include notification of blood bank due to increased hemorrhage risk and availability of adequate blood products, as recommended by the American College of Radiology 2
A multidisciplinary team approach is recommended if placenta accreta spectrum is suspected, as suggested by the American College of Radiology 5, 2

Women with prior cesarean deliveries and anterior low-lying placenta require careful evaluation for placenta accreta spectrum disorder, as recommended by the American College of Radiology 5, 6, 2
Risk of placenta accreta increases 7-fold after one prior cesarean delivery and up to 56-fold after three cesarean deliveries, according to the American College of Radiology 7, 2
MRI may be helpful in difficult cases or when placenta accreta is suspected based on ultrasound findings, as suggested by the American College of Radiology 6, 2

Failure to perform transvaginal ultrasound for accurate measurement of IOD, as warned by the American College of Radiology 1, 3
Digital examination before excluding placenta previa, which can trigger hemorrhage, as warned by the American College of Radiology 2
Missing placenta accreta spectrum in women with anterior low-lying placenta and prior cesarean deliveries, as warned by the American College of Radiology 5, 6

The American College of Radiology recommends scheduling follow-up ultrasound at 32 weeks gestation to re-evaluate the evolving relationship between the placental edge and internal cervical os 8
The follow-up ultrasound should evaluate placental edge-to-internal os distance using transvaginal ultrasound for precise measurement 8
The follow-up ultrasound should evaluate placental location and morphology, including assessment for any concerning features of placenta accreta spectrum disorder 8
Cervical length should be assessed to evaluate preterm delivery risk 9
Doppler evaluation should be performed to identify any vessels overlying the internal cervical os (vasa previa) 9

If anterior placenta with prior cesarean, carefully evaluate for placenta accreta spectrum disorder features including loss of retroplacental clear space, placental lacunae, and bladder wall abnormalities 10, 8

The American College of Radiology recommends a moderately full bladder (200-300 mL) for optimal evaluation of the lower uterine segment, using a high-frequency linear probe (5-9 MHz) if the patient's body habitus allows, and evaluating the retroplacentary clear zone without excessive probe pressure to avoid artifacts 11, 12
A empty bladder can impede adequate evaluation of the interruption of the vesical wall, placentary protrusion, and uterovesical hypervascularization 11, 12

The American College of Radiology suggests that transvaginal ultrasound should be used in conjunction with transabdominal ultrasound, particularly for evaluating the myometrium of the lower uterine segment, the placenta, and the myometrio-placental interface, providing higher resolution evaluation 11, 12, 13

The American College of Radiology recommends initial screening typically during the second trimester anatomy scan at 18-22 weeks, with follow-up evaluation at 28-32 weeks to reassess placental position 11, 12

The American College of Radiology identifies lagunas intraplacentarias (sonolucent spaces with slow or turbulent flow) as the finding with the highest sensitivity and positive predictive value for placental accreta spectrum, as well as loss of the normal retroplacentary hypoechoic zone, reduced myometrial thickness (<1 cm), placentary protrusion, and bladder wall abnormalities 11, 12, 13
Doppler color findings suggestive of accreta spectrum include lagunas placentarias, abnormal Doppler color patterns, increased placental vascular flow, subplacental vascularization, and vascularization at the uterine-serosal interface 11, 12

The American College of Radiology notes that isolated loss of the retroplacentary zone has a sensitivity of only 52% and specificity of 57%, with a false positive rate of 21%, as it can be absent in normal anterior placentas 11, 12, 13
Applying excessive pressure with the probe during transabdominal evaluation can cause artifactural loss of the retroplacentary zone, and evaluating with an empty bladder can impede adequate assessment of the lower uterine segment 11, 12