Fetal ossification centers as predictors of gestational age in normal and abnormal pregnancies.

The main fetal ossification centers appear ultrasonically as egg-shaped echo-rich areas. The calcaneal and talar ossification centers are seen at the level of the tarsus osseus, and the distal femoral epiphyseal and proximal tibial epiphyseal ossification centers are found at the level of the knee. Examination of 312 normal pregnancies between 20 and 40 weeks of gestation showed that the calcaneal ossification center was detectable for 24 weeks of gestation, the talar ossification center from 26 weeks, and the distal femoral epiphyseal and proximal tibial epiphyseal ossification centers, from 32 and 36 weeks, respectively. Corresponding figures found for 36 pregnancies showing intrauterine growth retardation (IUGR), examined between 34 and 40 weeks of gestation, were similar for the calcaneal and talar ossification centers but showed delays in the development of the epiphyseal ossification centers, which were particularly striking in cases of symmetrical IUGR. The amniotic fluid lecithin/sphingomyelin ratio was also evaluated in 51 normal pregnancies between 31 and 38 weeks of gestation and was found to be greater than or equal to 2 in every case where the distal femoral epiphyseal ossification center was greater than or equal to 6 mm in diameter. Evaluation of fetal ossification centers may be another useful means to evaluate gestational age in late pregnancy