Placental development and sonographic evolution

A low-resistance exchange interface connects the fetal and maternal components of the placenta. The fetal surface is usually smooth on ultrasound and is connected to the chorionic plate and amniotic membrane. Maternal circulation enters the intervillous spaces at the basal plate and decidua basalis, where the maternal surface is located next to the myometrium (Herrick & Bordoni, 2023; Kapila & Chaudhry, 2023).

The chorion frondosum gives rise to chorionic villi, which extend into maternal blood-filled intervillous spaces. This interface facilitates the return of fetal waste to the mother's circulation while also delivering nutrients and oxygen to the fetus. The functional lobules of the term placenta are represented by the cotyledons that make up the maternal surface of the placenta (Herrick & Bordoni, 2023; Kapila & Chaudhry, 2023).

Figure 1. Maternal and fetal placental layers.

Placental grading (0-III): timing and appearance

The classic Grannum grading system assesses placental maturation in three major zones: the chorionic plate, the placental substance, and the basal plate. Although placental grading can be useful as a descriptive ultrasound tool, the expected timing varies throughout gestation. As a result, the grade should be documented descriptively and interpreted within the larger clinical context rather than used as a stand-alone assessment of placental function (Grannum et al., 1979; Grannum & Hobbins, 1982).

Figure 2. Classic Grannum placental grades with progressively greater maturity/calcification pattern from Grade 0 to Grade III.

Grade	Typical gestational context	Key sonographic appearance
0	Usually earlier gestation, often before ~31 weeks	Homogeneous placental body, smooth/even chorionic plate, and little to no visible calcification.
I	Common in later 2nd to early 3rd trimester	A few small echogenic foci within the placental body and early shallow undulation/indentation of the chorionic plate.
II	Common in the later 3rd trimester	More numerous echogenic densities, comma-like chorionic plate indentations extending into the placenta but not to the basal plate, and increasing basal linear echoes.
III	Usually near term or post-dates; concerning if clearly preterm	Complete chorionic plate indentations to the basal plate with cotyledon formation and numerous coarse calcifications.

Reporting pearl. If different placental segments appear to have different grades, report the highest grade identified and note whether the change is focal or diffuse.

Calcifications, septations, and lakes: benign vs concerning patterns

Not every echogenic focus or sonolucent placental space represents pathology. A key practice point is to separate expected placental maturation from findings that warrant clinical correlation with gestational age, fetal growth, Doppler findings, placental location, maternal history, or placenta accreta spectrum risk factors (American College of Obstetricians and Gynecologists [ACOG], 2021; American College of Obstetricians and Gynecologists [ACOG] & Society for Maternal-Fetal Medicine [SMFM], 2018; Keller et al., 2024).

Pattern	Usually reassuring when...	Pause and correlate when...
Focal calcifications	Scattered echogenic foci appear later in gestation and fetal growth, and Doppler remains normal.	Calcifications are coarse, diffuse, or accompanied by clearly preterm Grade III-type maturation, fetal growth restriction, hypertensive disease, or abnormal umbilical artery Doppler.
Septations/indentations	Shallow to moderate chorionic plate indentations evolve gradually with advancing gestation.	Indentations appear complete unusually early, or the placenta also shows coarse calcification and abnormal texture.
Placental lakes	A few smooth anechoic spaces are seen in an otherwise normal placenta without a low-lying placenta or PAS risk pattern.	There are many or very large spaces, the placenta appears thickened, or the sonolucent spaces are irregular and seen in a scarred lower uterine segment.
Lacunae / PAS-type spaces	This term should not be used loosely for ordinary placental lakes.	Irregular "moth-eaten" intraplacental spaces, feeder vessels, turbulent Doppler flow, previa/low-lying placenta, or prior cesarean delivery raise concern for PAS.

Avoid Overcalling Normal Findings: Physiologic placental lakes and late gestational calcifications are commonly overcalled. Greater concern arises when findings appear unexpectedly early, are diffuse, or are accompanied by fetal growth abnormalities, abnormal Doppler findings, or placenta accreta spectrum risk features (ACOG, 2021; ACOG & SMFM, 2018; Keller et al., 2024).

Common pitfalls and mimics

Several interpretation pitfalls can result in overdiagnosis of placental disease. Calcification alone should not be used to diagnose "premature placental aging" without taking into account gestational age, fetal growth, maternal history, and Doppler findings (ACOG, 2021; Grannum et al. 1979). Smooth placental lakes should not be confused with irregular PAS-type lacunae, particularly in patients with placenta previa, a low-lying placenta, or a history of cesarean delivery (ACOG & SMFM, 2018; Keller et al., 2024).

Other pitfalls include assigning an overall placental grade based on a poorly visualized area, using imaging settings that overemphasize echogenicity, ignoring the highest grade present when different placental regions have different appearances, and equating placental grade with function. A transient uterine contraction can also alter the local placental contour; if the appearance is unusual, rescanning after the contraction ends can help avoid misinterpretation.

Placental Survey: Ultrasound Assessment Steps

A structured placental survey supports consistent documentation and reduces unnecessary follow-up. The standard obstetric ultrasound examination includes assessment of placental location, appearance, and relationship to the cervix when clinically relevant (American Institute of Ultrasound in Medicine [AIUM], 2024).

Step 1: Identify the placental location and its relationship to the internal os and any uterine scar.

Step 2: Evaluate the fetal surface for smoothness and assess the chorionic plate.

Step 3: Assess the placental body for texture, focal sonolucent spaces, and thickness if the placenta appears abnormal.

Step 4: Examine the maternal side adjacent to the myometrium, including the retroplacental region and cotyledon pattern.

Step 5: Correlate findings with gestational age and document clearly using descriptive language.

Reporting language and appropriate follow-up recommendations

A standardized report improves communication and reduces unnecessary follow-up. When using placental grading, the grade should be paired with key descriptors rather than reported as a number alone. Important reporting elements include placental location, relationship to the internal os, grade if used by the department, calcifications, chorionic plate indentations, cotyledon pattern, placental lakes or irregular lacunae, placental thickness if subjectively abnormal, and maternal-fetal interface observations when relevant (AIUM, 2024).

When findings are unexpectedly advanced for gestational age or when fetal growth or Doppler findings are abnormal, the report should recommend appropriate clinical correlation rather than diagnosing placental insufficiency based on grade alone (ACOG, 2021; Grannum et al., 1979).

Scenario	Sample reporting language
Expected late gestational maturation	Posterior placenta, Grade II. Mild scattered placental calcifications and moderate chorionic plate indentations are present, compatible with expected late gestational maturation. No focal retroplacental abnormality identified.
Incidental placental lakes	Anterior placenta with a few smooth intraplacental lakes. No low-lying placenta and no additional sonographic markers of placenta accreta spectrum were identified on this examination.
Definite preterm advanced placental maturation	The placenta demonstrates Grade III-type maturation with complete chorionic plate indentations and coarse calcifications before term. Recommend clinical correlation with fetal growth and umbilical artery Doppler in the appropriate setting.

Clinical Follow-Up Approach

Finding pattern	Suggested interpretation / next step
Expected maturation for gestational age	Document descriptively; additional imaging is not recommended based on placental grade alone.
Definitive preterm Grade III-type change or diffuse coarse calcification	Correlate with fetal growth, maternal hypertensive disease/smoking history, and umbilical artery Doppler when clinically indicated.
Irregular lacunae in a low-lying or previa placenta with a scar history	Evaluate for placenta accreta spectrum using institutional and ACOG/SMFM guidance.

Umbilical artery Doppler and placental insufficiency

Figure 3. Normal Umbilical Artery Waveform

The umbilical artery waveform reflects downstream placental resistance. In a normally functioning placental bed, the circulation is low resistance and maintains forward end-diastolic flow. As placental resistance rises, diastolic flow decreases, and Doppler indices such as the systolic/diastolic ratio, resistive index, and pulsatility index increase. Severe placental disease may progress to absent or reversed end-diastolic flow (ACOG, 2021).

Figure 4. Conceptual umbilical artery Doppler diagram showing how increased placental resistance lowers end-diastolic flow.

Doppler should be used to add functional information when placental maturation appears unexpectedly advanced. An increasing systolic/diastolic ratio may support rising placental resistance, but it must be interpreted using gestational age-specific norms and the complete clinical picture. Placental grade alone does not diagnose placental insufficiency (ACOG, 2021; Grannum et al., 1979).

Ultrasound history and the evolution of placental grading

Placental grading is rooted in classic obstetric ultrasound literature. Grannum and colleagues introduced a practical Grade 0–III placental classification in 1979, describing progressive ultrasound changes associated with placental maturation (Grannum et al., 1979). Subsequent literature further clarified the sonographic appearance of the placenta and the clinical limitations of relying on placental grade as a stand-alone assessment (Grannum & Hobbins, 1982; Hill et al., 1983).

Contemporary practice treats placental grade as a descriptive observation that should be interpreted with clinical correlation rather than as an isolated fetal well-being test. This approach is consistent with modern obstetric ultrasound practice, which emphasizes placental location, appearance, fetal growth, Doppler findings when indicated, and clinical risk factors (ACOG, 2021; AIUM, 2024).

Figure 5. Short teaching timeline from early obstetric ultrasound to modern contextual use of placental grading.

Key Learning Takeaways

Start with anatomy: the chorionic plate is located on the fetal side, while the basal plate and decidua basalis are located on the maternal side adjacent to the myometrium.
Grade the placenta by assessing the chorionic plate, placental body, and basal plate together.
Normal placental maturation overlaps across gestation. Late calcification does not automatically indicate placental insufficiency.
Grade III-type change is more concerning when it is clearly preterm or when paired with fetal growth restriction, maternal hypertensive disease, or abnormal umbilical artery Doppler findings.
Use standardized reporting language to describe what is seen instead of overcalling “premature placental aging.”

Placental Grading Across Pregnancy: Normal Maturation vs Pathologic Change

Chapter 1