The cytogenetic normality and developmental competence of mouse oocytes derived from prepubertal females were investigated to determine ooplasmic maturity after in vitro fertilization (IVF) and to examine the ability of the resultant blastocysts to develop to term. To estimate the effect of body weight and age on ovulation, prepubertal female (BALB/c × C57BL/6J) F₁ mice, 20–30 days of age, were classified into two groups according to body weight as follows: a light group (L) of 8.5–12 g and a heavy (H) group of 13–16 g. The IVF blastocysts were fixed as chromosome samples, and some of the blastocysts were transferred to pseudopregnant recipients. The implantation rates and number of fetuses were subsequently evaluated at 20 days postcoitus. The average number of ovulated oocytes differed significantly, with 46.2, 18.1, and 37.1 in the H, L, and control groups, respectively. All fertilization rates were high, and there were no significant differences. However, 16.8% of zygotes from the L group were arrested at the 1-cell stage, with mostly male premature condensed chromosomes (PCC). The rate of development to the blastocyst stage was significantly low in the L group (36.6%). The rates of implantation and development to newborns were significantly lower in the H group than in the pubertal mice.