First-Trimester Ultrasound Volumetry

Measurement Techniques and Potential Application in the Prediction of Pregnancy Complications

Kwok Yin Leung; Teresa Ma; Betty YT Lau; Min Chen

Disclosures

Expert Rev of Obstet Gynecol. 2012;7(4):379-386. 

In This Article

EV or FV

EV/FV can be measured directly[2] or by subtracting the amniotic fluid and yolk sac volumes from the GSV.[48] Using a direct method, the fetus is traced by drawing a contour line along its head and trunk while excluding the limbs, which usually cross over each other or touch the face in the late first trimester.[8,49] However, other investigators have suggested that measurement of FV should include the limbs, which represent a significant proportion (8–10%) of the size of the embryonic/fetal body.[50] Specialized 3D software that allows the combination of several volume measurements from various anatomical regions of the fetus in the same dataset at the same time is required. The head volume can also be measured separately and then subtracted from the total head and trunk volume to obtain the volume of the fetal trunk.[19]

VOCAL is a commonly used technique,[2,8] but it is more difficult and it is time consuming to measure embryonic limbs, which appear as separate disconnected objects from the trunk in certain planes unless a thin connecting stalk is drawn between the limbs and the trunk. The reproducibility of doing so has not yet been proven. VOCAL with 9° rotation provides the best compromise among validity, reliability and time required for measurements when compared with the 30°, 15° and 6° rotation steps.[39] Other described methods include the multiplanar technique and, rarely, the XI VOCAL technique.[42] It appears that 3D ultrasound (using a multiplanar, VOCAL or XI VOCAL technique) can provide a reproducible measurement of the fetal trunk and head volume at 11 + 0 to 13 + 6 weeks' gestation.[42] The inter- and intra-observer correlations have shown to be very good.[7,10,42]

Semiautomated techniques, using both VOCAL and SonoAVC,[51] facilitate the measurement of the embryo without the need to physically define its contour, which is the prime limiting factor in the aforementioned techniques. However, there was a significant difference in the volumetry between this semiautomated technique and the conventional VOCAL technique alone with 9° rotations.[51]

There was a significant correlation between EV and GA or CRL,[2,7,16] with a linear association between 11 + 0 weeks and 13 + 6 weeks,[8] or between a CRL of 45 and 84 mm.[8] These findings are consistent with previous 2D sonographic studies on the S-shaped pattern of fetal growth with gestation, with the linear component at 10 and 30 weeks.[52]

At between 11 + 0 and 13 + 6 weeks there is a five- to six-fold increase in FV but only a doubling in CRL.[8] This can represent a mathematical relation of quadratic versus cubic measurement. Whether FV is a potential first-trimester marker of IUGR is not clear. There was no difference in EV between male and female embryos.[1] EV was a better predictor of GA than CRL in a study of 30 in vitro fertilization pregnancies between 7 and 10 weeks of gestation.[53]

A recent review has shown that a wide discrepancy exists in reported normal volumes of first-trimester embryos, ranging from 0.2–0.23 cm3 at 7 weeks to 3.91–5.12 cm3 at 10 weeks.[7,10,54] This wide discrepancy is likely to be due to inconsistencies in 3D volumetric methodology, inadequate assessment of method repeatability and validity, and a diversity of mutually incompatible 3D imaging formats and software measuring tools. Standardization of the 3D volumetric methodology will help to improve quality assurance in fetal volumetry, and then facilitate its clinical use.[54]

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