After automatic acquisition of 3D volume, the placenta is defined by the basal and chorionic border with the uterine wall carefully excluded. Common measurement techniques include the VOCAL method with a 30° rotation angle[10,22] or the multiplanar method. Inter- and intra-observer correlations were good, ranging from 0.8 to 0.99 in some studies,[10,22,23] but relatively poor in another study. XI VOCAL cannot be used interchangeably with VOCAL or multiplanar techniques in measuring PV at 11–14 weeks' gestation because of inferior reproducibility and in vivo validity.
There was a strong correlation between PV and CRL or GA from 7 to 13 + 6 weeks.[4,16,18] PV increased from 1.7 ml at 7 weeks, through 42.6 ml at 10 + 6 weeks, to 91 ml at 13 + 6 weeks.[4,18] While CRL increased only 4.4-times from 9 to 40 mm, the mean PV increased 10.5-times.
Placental quotient (PQ) equals PV divided by the fetal CRL, and is a simple first-trimester parameter to indicate whether a placenta is large or small for a given fetus. Neither PV nor PQ are associated with maternal plasma cell-free fetal DNA levels, which may reflect the extent of placental apoptosis or other unknown mechanisms.
There are limitations in using placental volumetry for clinical applications because of the physiological variations in placental shape, weight and volume at each stage of gestation, the heterogeneous nature of placental growth, and the reproducibility and accuracy of measuring the volume of the placenta, an irregular structure.
Expert Rev of Obstet Gynecol. 2012;7(4):379-386. © 2012 Expert Reviews Ltd.