Smith–Magenis syndrome (SMS, OMIM: 182290) is a multiple congenital anomalies and intellectual disability syndrome due to a 3.45 Mb microdeletion involving 17p11.2 and is estimated to occur about one in 25,000 births. Up to now, the ultrasound findings of the foetus with SMS and their external genital defects in patients are rarely reported. This case indicates that foetus with SMS may presentpolyhydramnios and ventriculomegaly in the second trimester. The newborn male patient had an abnormal phenotype in which he has micropenis and his anus is close to the perineal body. The identification of this case may further expand the phenotypic spectrum of this genetic disorder.