AbstractThe increasing use of non‐human primates to study fetal development and neonatal management has necessitated the availability of fetuses of known gestational history. In this study, prenatal development and growth were investigated in the common marmoset (Callithrix jacchus) using ultrasound. The objectives of this study were: (1) to determine the accuracy of ultrasound for monitoring prenatal growth and development in common marmosets, (2) to determine if litter size influences prenatal growth trajectories, and (3) to assess growth discordancy among litter mates. Fifty pregnancies were monitored longitudinally using real‐time abdominal sonography. During each examination the number of fetuses was recorded, and crown‐rump length (CRL) and biparietal diameter (BPD) were measured. The results indicate that ultrasound is a reliable method for observation of gross morphological changes during prenatal development in this species. Measures of CRL and BPD taken early in gestation using ultrasound were in agreement with those from gross specimens. Triplets were significantly (P<0.05) smaller than twins for both BPD and CRL. No significant relationship was found between litter size and within litter variation in CRL or BPD. This study is the first longitudinal investigation of prenatal growth and development inC. jacchus. The observations from this study will be of use for determining approximate gestational age of fetuses, as well as providing guidelines for routine monitoring of pregnancy in this species. © 1995 Wiley‐L