Frozen-thawed embryo transfers, whose number has risen considerably in recent years, reportedly result in heavier birth weights than fresh embryo transfers. To find out what this difference means and the stage at which it becomes manifest during fetal development, we studied birth weight and gestational sac size, which reflects development immediately following implantation, in 365 single pregnancies employing fresh embryo transfer and 227 employing frozen-thawed embryo transfer. Comparison of fresh embryo transfers and frozen-thawed embryo transfers revealed that average birth weights were significantly higher in the latter, with average values ± SD of 2896.0 ± 515.7 g and 3060.0 ± 529.2 g, respectively. Transvaginal ultrasound showed significantly larger average gestational sac diameters at 21, 22, 23, 28, 29 and 30 days after fertilization in frozen-thawed embryo transfers. We speculate these results are explained mainly by hormone replacement therapy in frozen-thawed embryo transfer cycles exerting a more positive influence on the endometrium, promoting smoother implantation, greater development during early pregnancy, and significant increases in birth weight. Amidst concerns regarding the impact exerted on fetuses by the artificial operations entailed by in vitro fertilization and embryo transfers, these findings may serve as evidence of the safety of frozen-thawed embryo transfers.