The rhesus monkey has played an important role in the history of reproductive research. Because of limitations on the exportation and availability of this species, several other species of nonhuman primates have been considered as alternate models. Among them is the crab-eating, or cynomolgus, macaque (Macaca fascicularis), which displays similarities in developmental, reproductive, and physiological parameters. The use of both species of macaques for pregnancy-related studies necessitates the assessment of differences in growth and development through gestation. Observations during the embryonic and fetal periods in both species have been compared with diagnostic ultrasound. Results indicate no significant differences in size during the embryonic and early fetal periods, but a greater acceleration of growth in the rhesus begins at approximately 100–110 gestational days (GD). Analysis of embryonic and fetal heart rates indicate no differences between the two species. Normal predictive values for a variety of growth parameters including gestational sac, greatest length, biparietal diameter, and femur length have boon calculated by multiple regression analysis. These charts have proven useful for confirming the gestational age after timed matings and for predicting the age of animals for which the conception date is not known.