Prenatal diagnosis of isolated abnormal number of ribs

Objective To determine the incidence of prenatal isolated abnormal number of ribs, using three-dimensional (3D) ultrasound. Methods This was a prospective study of low-risk women with singleton pregnancy and normal fetal anatomy scan. 3D examinations of the fetal ribs were carried out at the time of the routine anomaly scan at 14-16 or 20-24 weeks of gestation. A 3D volume of the spine was acquired with a mechanical sweep that lasted between 2 and 4 s per volume and was performed when the fetus had a minimally flexed head position. Fetal ribs were reviewed on the rendered image (maximal mode), both while the patient was in the clinic and again later. Results Three hundred and sixty-seven pregnant women were evaluated. Twenty-three (6.3%) fetuses were found to have an abnormal number of ribs. Fifteen (4.1%) had 11 ribs unilaterally, with the last (12 ^th) rib missing. One patient (0.27%) had 13 ribs unilaterally (with a small extra rib after the last rib). Two (0.5%) patients had 13 ribs bilaterally with an extra rib after the 12^th rib on each side and two (0.5%) patients had 13 ribs bilaterally with an extra cervical rib on each side. Three (0.8%) patients had an extra unilateral cervical rib. In all, 1.4% of patients had extra cervical ribs. Conclusions The incidence of abnormal number of ribs in this normal population was 6.3%. When isolated, this finding is a normal variant.