Placenta, Cord, and Amniotic Fluid

Quick Answer: Map placenta to os (previa if covering). AFI ≤5 or DVP <2 = oligohydramnios. AFI >24 polyhydramnios context. Check cord insertion, 3 vessels, vasa previa with Doppler. Accreta = loss of clear zone, placental lacunae.

Placental Location and Terminology

Document placenta relative to internal cervical os on transverse view; transvaginal imaging improves accuracy near term.

Migration: Many low-lying placentas and previas resolve by third trimester—do not diagnose previa at 16 weeks without follow-up language. Third-trimester TV reassessment is standard for low-lying/placenta previa.

Worked Scenario: 28-Week Low-Lying Placenta

Placental edge 1.5 cm from internal os, not covering. Impression: low-lying placenta—repeat transvaginal assessment in third trimester per protocol.

Placenta Accreta Spectrum

Especially with anterior placenta previa and prior cesarean:

Sonography suggests accreta—MRI and surgical planning follow. Do not diagnose on color flash artifact alone—optimize settings.

Placental Variants

• Circumvallate placenta: raised chorionic plate edge—associated with growth restriction, abruption in some studies
• Bilobed/succenturiate lobe: accessory lobe connected by vessels—retained lobe and vasa previa risk if vessels cross os
• Marginal vs velamentous cord insertion: velamentous vessels lack Wharton's jelly protection

Umbilical Cord Assessment

Vasa previa suspected with velamentous insertion near cervix—color Doppler over internal os confirms unprotected fetal vessels.

Amniotic Fluid: AFI and Deepest Pocket

AFI Technique

Divide uterus into four quadrants (linea nigra and umbilicus). Measure deepest vertical pocket (DVP) in each without cord or fetal limb; sum four values.

DVP (Single Deepest Pocket)

Used in biophysical profile and some protocols to reduce false oligohydramnios when pockets are unequal. DVP <2 cm may define oligohydramnios in some guidelines.

Worked Scenario: Unequal Quadrants

One quadrant 8 cm, others 1 cm each—AFI may be borderline. Consider DVP method and clinical context (ROM, fetal renal anatomy, growth).

Oligohydramnios and Polyhydramnios Etiologies

Anhydramnios before mid-pregnancy suggests pulmonary hypoplasia risk (Potter sequence if renal etiology).

Placental Grading (Grannum)

Grades 0–III describe calcification/maturity—poor predictor of lung maturity alone. Grade III in late third trimester may be normal or associated with smoking, postdates—not isolated indication for delivery on registry.

Doppler Overlap (Growth Restriction)

Elevated umbilical artery S/D, absent or reversed end-diastolic flow indicates severe placental insufficiency—pair with biometry and fluid for integrated reporting.

Exam Traps

• Previa at 16 weeks without follow-up caveat
• Counting cord in pocket as fluid
• Missing vasa previa with velamentous insertion near os
• AFI without excluding cord/limb from measurement
• Over-interpreting placental grade as fetal distress

The placenta-fluid-cord triad anchors high-risk OB vignettes—always link fluid volume to renal anatomy, membrane status, and twin pathology when applicable.

Placental Abruption Sonographic Clues

Abruption is primarily clinical diagnosis; ultrasound sensitivity is limited. Findings may include retroplacental hematoma, placental thickening, heterogeneous echotexture, and separation of placental edge. Normal ultrasound does not exclude abruption—registry items may test this limitation. Doppler is not diagnostic for abruption.

Subchorionic Hemorrhage

Hypoechoic or echogenic collection between chorion and uterine wall in first trimester often resolves. Large hematoma may associate with miscarriage risk early—document size and trend. Distinguish from circumvallate placenta (raised placental margin) in second trimester.

Twin-Specific Fluid Assessment

In monochorionic twins, measure fluid per sac when separate amniotic cavities exist. TTTS shows polyhydramnios in recipient (DVP often >8 cm) and oligohydramnios in donor (DVP <2 cm). Single sac monoamniotic twins cannot use AFI per twin—document single deepest pocket and cord entanglement concern.

Oligohydramnios Severity and Outcomes

Anhydramnios before mid-pregnancy from bilateral renal agenesis predicts pulmonary hypoplasia (Potter sequence). Late oligohydramnios from ruptured membranes or placental insufficiency has different management—correlate with membrane assessment, nitrazine/ferning if clinical ROM suspected, and renal anatomy.

Polyhydramnios Workup

Idiopathic polyhydramnios is diagnosis of exclusion. Evaluate fetal swallowing (esophageal atresia, facial clefts), diabetes, fetal anemia/hydrops, neurologic disorders, and twin complications. Stomach absent with polyhydramnios suggests swallowing obstruction.

Cord Cysts and Structural Variants

Umbilical cord cysts in first trimester may resolve; persistent cysts or cystic allantoic remnants warrant follow-up. Nuchal cord loops are common and usually benign—document number of loops if tight or multiple. Cord knot is difficult to confirm sonographically but may be suspected with color Doppler.

Placental Masses

Chorioangioma appears as well-circumscribed hypoechoic or mixed mass near chorionic plate with turbulent vascular flow—may cause polyhydramnios, fetal anemia, or growth restriction when large. Distinguish from subplacental myometrial fibroid by location and vascular pattern.