OBJECTIVE The aim of this study was to assess intra and interobserver reproducibility of placental volume and vascularization during the first trimester of pregnancy studied by three dimensional ultrasonography and angio power Doppler. METHODS This is a prospective study in 69 singleton pregnancies. Once the bi-dimensional protocol study was carried out, we performed a 3D-US (three-dimensional ultrasonography) of the placenta by abdominal ultrasonography. The Virtual Organ Computer-Aided Analysis program was used to evaluate the placental volume (PV), the placental quotient (PQ: placental volume/crown-rump length) and the vascular indices (vascularization index VI, flow index FI and vascularization-flow index VFI). The intraobserver and interobserver variability were respectively expressed as an intraclass correlation coefficient (Intra-CC) and interclass correlation coefficient (inter-CC). RESULTS Intraobserver correlation for PV was excellent with an Intra-CC of 0.97 while an Inter-CC of 0.71 demonstrated less agreement between observers. In the same way, PQ showed better intraobserver than interobserver correlation, with an Intra-CC of 0.97 and an inter-CC of 0.67. The analyzed vascular indices had both excellent intraobserver and interobserver correlation coefficients, with values of 0.98 and 0.96 for VI, 0.93 and 0.89 for FI and 0.97 and 0.95 for VFI, respectively. CONCLUSIONS Our study demonstrate an excellent intra and inter-observer reproducibility for vascular indices and a good reproducibility of the evaluated Doppler indices with intra-CC higher than 0.90. PV and PQ were also reproducible most of all within the same observer. As a conclusion, first trimester tridimensional sonography is a reproducible tool for the systematic study of placental vascularization.