Objective: To determine associations between fetal rib number anomalies detected on ultrasonography and chromosomal anomalies and other structural anomalies, and the outcome of affected pregnancies. Methods: All cases of fetal rib number anomalies referred to the Prenatal Diagnosis Clinic of Queen Elizabeth Hospital between 1 January 2016 and 31 December 2019 were reviewed. Fetal ribs were examined by static threedimensional multiplanar or volume contrast ultrasonography. Genetic counselling was offered. The prenatal and postnatal records were reviewed. Results: 21 fetuses with rib number anomalies were identified over 4 years. The most common presentation was unilateral or bilateral absence of the 12th thoracic rib (n=12, 57.1%), followed by the presence of lumbar rib (n=6, 28.6%) and the presence of cervical rib (n=3, 14.3%). Three (14.3%) fetuses were identified to have anomalies in other systems: unilateral absence of nasal bone (n=1) and minor vascular anomalies (n=2). One patient with multiple anomalies of the fetus underwent amniocentesis, and the chromosomal microarray analysis was normal. Postnatally, 13 babies had chest radiographs taken. Two were confirmed to have normal number of ribs. Prenatal and postnatal findings were consistent in 6 (46.2%) babies. Conclusion: Fetal rib number anomalies were an isolated finding in most cases. The prognosis is good in the absence of other major anomalies. The accuracy of prenatal ultrasonography appears to be low. These findings do not support routine counting of fetal rib number in second-trimester ultrasonography.