Placental Volume: 2D EPV vs 3D VOCAL™
A 30-second handheld measurement, checked against the gold standard in 58 paired scans.
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Summary
Placental volume has been proposed as a marker worth following in pregnancy, but the reference method for measuring it — 3D VOCAL™ — needs a 3D-capable machine, a trained operator, and time to acquire and process multiple images. The Estimated Placental Volume (EPV) method needs a handheld probe, three caliper measurements, and one equation, and returns a number in about 30 seconds. Kliman and colleagues compared the two directly in 58 women scanned once between 11 and 22 weeks, all of whom went on to deliver at term without major obstetric complications.
The correlation was strong (Pearson r = 0.93, p < 0.001) and the fitted slope was 1.028, meaning the 2D method did not systematically read high or low against the 3D method. Mean difference between the methods was 2.09 cm³ (95% CI −7.5 to 11.6). That is the reassuring half of the result, and it is the half most readers will take away.
The other half is the spread. The standard deviation of the differences was 36.3 cm³, which puts the 95% limits of agreement at −69.1 to +73.2 cm³ — a fixed window roughly 142 cm³ wide, independent of placental size. Applied to a 100 cm³ placenta that window is ±70% of the value; applied to a 300 cm³ placenta it is ±24%. Two of 58 observations fell outside the limits entirely. The infographic makes this scale-dependence visible, because it is the difference between a method fit for population comparison and a method fit for a decision about one patient.
Clinical bottom line
- Correlation and agreement are not the same question. r = 0.93 tells you the two methods move together; it tells you nothing about whether they return the same number on a given scan.
- The 95% limits of agreement span roughly 142 cm³ and do not scale with placental size — so the same absolute error is trivial for a large placenta and disqualifying for a small one.
- No systematic bias was found (mean difference 2.09 cm³, slope 1.028), which supports EPV for cohort-level and research use.
- The cohort was 58 uncomplicated term pregnancies, 91% African American, scanned once at 11–22 weeks. No abnormal placentas were studied.
- The study did not link placental volume to any pregnancy outcome. Feasibility was demonstrated; clinical utility was not.
Key references
- Kliman HJ, Romero R, Meyyazhagan A, Avila C, Chaiworapongsa T, Awonuga A, et al. Sonographic measurement of placental volume: a comparative analysis of two-dimensional and three-dimensional sonographic techniques. J Perinat Med. 2026;54(6):1061–4.Primary source. All figures in this infographic are taken from this paper.doi:10.1515/jpm-2026-0110 →
- Azpurua H, Funai EF, Coraluzzi LM, Doherty LF, Sasson IE, Kliman M, et al. Determination of placental weight using two-dimensional sonography and volumetric mathematic modeling. Am J Perinatol. 2010;27(2):151–5.Origin of the three-parameter convex-concave shell model behind EPV.
- Schwartz N, Oguz I, Wang J, Pouch A, Yushkevich N, Parameshwaran S, et al. Fully automated placental volume quantification from 3D ultrasound for prediction of small-for-gestational-age infants. J Ultrasound Med. 2022;41(6):1509–24.Source of the VOCAL™ 3D technique used as the reference standard.
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