Weights of viable trisomic mouse fetuses and placentas were compared with those of normal littermates between 12½ and 19½ days’ gestation. Mean trisomic fetal and placental weights were below normal weights at all stages. Differences in mean body weight were consistently significant, whereas those in placental weight varied. Trisomic fetuses of a specific karyotype predisposing to cleft palate were not smaller than trisomics of two other karyotypes, suggesting that a greater generalized growth lag was not responsible for the cleft palate. At each developmental stage, ratios of placental to body weights of normal and trisomic mice were similar. Sections of trisomic organs showed only one specific change: excessive degeneration of oocytes in some trisomic ovaries. All of these findings are discussed in relation to other studies of fetal and placental weight in mice and to studies of the effects of trisomy upon human development in ut