The objective of this study is to explore the effects of prenatal isolated enlarged cisterna magna (IECM) on postnatal development. We followed up 123 fetuses with an enlarged cisterna magna (ECM), who were divided into IECM (group 1) and non-IECM (ECM plus other anomalies, group 2) groups, and compared 60 normal infants with normal fetal ultrasound. We assessed infants postnatally using the Gesell Developmental Schedules. Fetal magnetic resonance imaging, karyotyping, and chromosomal microarray analysis test were offered. The developmental quotients of gross motor and adapting abilities were significantly lower in infants with IECM compared with normal infants (80.81±3.44 vs 99.20±4.54 and 98.70±3.27 vs 100.30±5.33, respectively), and the depths ≥15mm and 13 to 14.90mm were associated with significantly lower adapting abilities (P<0.03 and P<0.05) compared with the normal (97.16±2.17 and 98.21±1.76 vs 100.30±5.33, respectively), while these scores were still within the borderline range, and the other subtests were not significantly different. Furthermore, the group 2 showed a significantly worse performance in the test. The outcome of children with IECM may be guarded, especially some subtle deficits in adapting and gross motor abilities. In contrast, the developmental quotients of infants with prenatal non-IECM were lower than that those of normal infants. © 2017 John Wiley & Sons, Ltd.