The platyspondylic lethal skeletal dysplasias (PLSD) are a group of heterogeneous disorders including thanatophoric dysplasia (TD) and the TD variants (San Diego, Torrance, and Luton types). TD is the most common form and has been divided into two subtypes (TD1 and TD2) based on clinical and radiologic criteria and analysis of mutations in the fibroblast growth factor receptor 3 (FGFR3) gene. The variants are distinguished from TD by characteristic radiographic and chondro-osseous morphologic features. We have recently identified FGFR3 mutations in PLSD-San Diego type (PLSD-SD) which are identical to those found in TD1, but the known TD FGFR3 mutations were not found in the other PLSD variants. After reviewing radiographs from 32 cases of PLSD-SD and 47 cases of TD with gestational ages under 24 weeks, we noted novel accessory ossification centers in the ischia of 18 cases of PLSD-SD and 44 of TD, and the ilia in 18 cases of PLSD-SD and 20 of TD. Only three cases of TD and five cases of PLSD-SD did not have extra pelvic ossification centers. At a gestational age greater than 24 weeks, the extra centers are fused with the main bone. The radiographic appearance and chondro-osseous morphology of cases with and without accessory pelvic ossification centers were otherwise indistinguishable. Morphologically, the accessory pelvic ossification centers resulted from membranous ossification. Extra pelvic ossifications are a common radiographic finding in TD and PLSD-SD.