Skeletal abnormalities in fetuses with Down's syndrome: a radiographic post-mortem study

Objective. To evaluate skeletal abnormalities on post-mortem radiographs of fetuses with Down's syndrome. Materials and methods. Biometrical and morphological criteria, which are us ed for US prenatal detection of trisomy 21, were assessed. Limb long bones, biparietal diameter (BPD)/occipito-frontal diameter (OFD) ratio, ossificat ion of nasal bones and appearance of the middle phalanx of the fifth digit (P2) in 60 fetuses with Down's syndrome were analysed and compared with 82 normal fetuses matched for gestational age (GA) from 15 to 40 weeks' gestat ion (WG). Results. We observed reduced growth velocity of limb long bones during the third trimester in both groups, but the reduction was more pronounced in th e trisomic group. Brachycephaly was found as early as 15 WG in Down's syndr ome and continued throughout gestation (sensitivity 0.28, specificity 1). O ssification of the nasal bones, which can be detected in normal fetuses fro m 14 WG, was absent in one quarter of trisomic fetuses, regardless of GA. T he middle phalanx of the fifth digit was evaluated by comparison with the d istal phalanx (P3) of the same digit. We found that P2 was not ossified in 11/31 trisomic fetuses before 23 WG, and was either not ossified or hypopla stic in 17/29 cases after 24 WG (sensitivity 0.56, specificity 1). Conclusions. Three key skeletal signs were present in trisomic fetuses: bra chycephaly, absence of nasal bone ossification, and hypoplasia of the middl e phalanx of the fifth digit. All these signs are appropriate to prenatal U S screening. When present, they fully justify determination of the fetal ka ryotype by amniocentesis.