Placenta previa is an abnormal implantation condition where the placenta completely or partially covers or lies near the internal cervical os. Its incidence is approximately 1/200 pregnancies, with prior cesarean section, multiparity, advanced maternal age, and multiple gestation being risk factors. Three types are defined: complete (fully covers the os), marginal (reaches the os edge), and low-lying (within 2 cm of os but not covering it). Transvaginal ultrasound (TVUS) is the gold standard for diagnosis — far more accurate than transabdominal US. A significant proportion of placenta previa detected in the second trimester resolves by the third trimester with lower uterine segment development ('placental migration'). Clinically presents with painless vaginal bleeding; complete previa is an indication for cesarean delivery. The risk of association with placenta accreta spectrum is particularly high in cases implanted over prior cesarean scar.
Age Range
18-45
Peak Age
32
Gender
Female predominant
Prevalence
Uncommon
Placenta previa develops from blastocyst implantation on the decidua basalis of the lower uterine segment. Normal implantation occurs in the fundus or corpus region; however, endometrial damage (prior cesarean, curettage, endometritis), insufficient vascularization, or need for large placental surface area (multiple gestation) directs the placenta to the lower segment. Coverage of the internal os by the placenta causes separation of villous tissue from decidual attachments during cervical dilation, creating the painless bleeding mechanism — bleeding originates from maternal vascular sinuses, fetal blood loss is rare. On ultrasound, placental tissue is visualized partially or completely covering the internal os; an overdistended bladder on transabdominal US can cause false-positive results by approximating anterior and posterior uterine walls, distorting the cervix. TVUS eliminates this artifact. 'Placental migration' is not true displacement — it represents development and elongation of the lower uterine segment as pregnancy progresses, causing the placental edge to move away from the os; therefore, second-trimester diagnosis should be re-evaluated in the third trimester.
Placental tissue completely or partially covering the internal cervical os on TVUS — pathognomonic ultrasonographic finding for previa diagnosis. Distance to the os is measured in millimeters and previa type is classified.
On transvaginal ultrasound, placental tissue is visualized overlying the internal cervical os. In complete previa, the placenta fully covers the os; in marginal previa, the placental edge reaches the os; in low-lying placenta, it is within 2 cm but does not cover. Placental tissue appears homogeneous, moderately echogenic, and its relationship to the cervical canal is clearly evaluated. TVUS provides optimal visualization of the cervical canal and internal os.
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On transvaginal ultrasound, placental tissue is seen completely/partially covering the internal cervical os, consistent with placenta previa.
Color Doppler ultrasound demonstrates placental vascularity over the internal os — showing spiral artery flow into the placenta. This finding supports the diagnosis of placenta previa and also allows evaluation for accreta spectrum. Abnormal vascularity (turbulent flow within lacunae, vessels extending to cervix) suggests accreta association.
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Color Doppler examination demonstrates placental vascularity over the internal os, supporting the diagnosis of previa.
On T2-weighted MRI, the placenta demonstrates intermediate-to-high signal intensity and its relationship to the internal os is clearly evaluated. The cervix appears as low T2 signal fibrous tissue — providing high placenta-cervix contrast. MRI is particularly complementary for posterior placenta previa, obese patients, or cases with inadequate US evaluation. Myometrial thickness and T2 hypointense bands are also assessed for accreta spectrum association.
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On T2-weighted MRI, placental tissue is seen extending to/covering the internal os, consistent with the diagnosis of placenta previa.
Cervical length measurement on TVUS is important in placenta previa management. Short cervix (<25 mm) increases risk of emergent bleeding and preterm delivery. Additionally, the placental edge-to-internal os distance is measured in millimeters: >20 mm = not low-lying (normal), 0-20 mm = low-lying, 0 mm = marginal, os covered = complete previa.
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Cervical length measures __ mm on TVUS with a placental edge-to-internal os distance of __ mm.
In cases of low-lying placenta or marginal previa detected in the second trimester, follow-up TVUS at 32-36 weeks can demonstrate the placenta moving away from the internal os ('placental migration'). Migration rate is low in complete previa (5-10%); in marginal and low-lying cases, resolution occurs in up to 80-90%. Migration is less expected in anteriorly located placenta implanted over prior cesarean scar.
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Follow-up TVUS demonstrates the placental edge moving away from the internal os, consistent with placental migration.
On T1-weighted MRI, retroplacental hemorrhage may appear as high signal intensity (methemoglobin). MRI's advantage over US in evaluating hemorrhagic complications in placenta previa cases is better imaging in posterior placenta or obese patients. Hematoma size and localization are valuable for surgical planning.
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High signal intensity in the retroplacental area associated with placenta previa is seen on T1-weighted MRI, consistent with subacute hemorrhage.
Criteria
Placenta completely covers the internal os
Distinct Features
Highest bleeding risk, cesarean mandatory, low migration rate (5-10%), highest accreta association risk
Criteria
Placental edge reaches but does not cover the internal os
Distinct Features
High migration chance (80-90%), vaginal delivery may be possible in some cases, moderate bleeding risk
Criteria
Placental edge within 2 cm of internal os but not reaching it
Distinct Features
Highest migration rate, vaginal delivery possible in most cases, low bleeding risk, follow-up US recommended
Distinguishing Feature
Accreta spectrum shows myometrial thinning, lacunae, and loss of clear space; in isolated previa, myometrial thickness is preserved
Distinguishing Feature
Abruption shows painful bleeding + retroplacental hematoma; previa bleeding is painless with placenta over the os
Distinguishing Feature
Subchorionic hematoma has normally positioned placenta with collection between chorionic membrane and uterine wall; in previa, the placenta is over the os
Distinguishing Feature
In vasa previa, placenta does not cover the os but fetal vessels cross the internal os; in previa, placental tissue is over the os
Urgency
urgentManagement
surgicalBiopsy
Not NeededFollow-up
specialist-referralComplete placenta previa requires cesarean delivery (planned at 35-37 weeks). In marginal/low-lying cases, follow-up TVUS at 32-36 weeks evaluates migration. Bleeding episodes require hospitalization, RhD prophylaxis, and fetal monitoring. Accreta spectrum association should be assessed with detailed US and MRI if needed; multidisciplinary approach (obstetrics + interventional radiology + anesthesia + urology) should be planned.
Placenta previa is the leading cause of third-trimester bleeding. Painless, bright red vaginal bleeding is characteristic. Complete previa mandates cesarean delivery. The majority of low-lying placentas detected in the second trimester migrate upward with lower uterine segment growth. Placenta previa combined with prior cesarean history carries high risk for accreta spectrum.