Objective To review systematically the literature on diagnostic tests and performance of second-trimester sonographic assessment of nasal bone (NB)

Clinical prediction rules allow for the integration of second-trimester ultrasound findings to calculate the risk of DS. However, results from the different clinical prediction rules are not concordant and may generate situations of overestimation or underestimation of the risk of DS. It is thus necessary to validate these clinical prediction rules externally to decide which is most suitable for clinical use.