It has previously been reported that ovine embryos cultured in Synthetic Oviduct Fluid medium supplemented with 20% human serum (SOF+HS) develop into lambs with a high birth weight. We have investigated this phenomenon by culturing ovine zygotes in SOF+HS or a serum-free version of Synthetic Oviduct Fluid with BSA and amino acids (SOFaaBSA) in place of serum. Zygotes were either obtained from superovulated and naturally mated ewes or produced in vitro. Embryos were subsequently transferred to synchronized recipient ewes (n = 63). An additional group of ewes (n = 16) served as flock fertility and lambing controls. Development of zygotes to stages suitable for transfer (i.e., good to excellent compact morulae or blastocysts) was not affected by medium (SOFaaBSA = 53 ± 5% vs. SOF+HS = 59 ± 5%) but was affected by source (in vivo-derived = 74 ± 5% vs. in vitro-derived = 35 ± 5%, p < 0.001). Embryos incubated in SOF+HS were morphologically different from those incubated in SOFaaBSA, having abundant lipid droplets. Pregnancy rate (65%) and embryo survival (48%) of recipients determined by ultrasonography on approximately Day 60 of pregnancy did not differ between medium treatments or source of embryo. Mean weight of lambs from embryos cultured in SOF+HS (4.2 ± 0.2 kg) was significantly heavier than that of controls (3.4 ± 0.2 kg, p < 0.01) or of lambs from embryos cultured in SOFaaBSA (3.5 ± 0.2 kg, p < 0.05). Furthermore, mean gestation length was longer in recipients receiving embryos incubated in SOF+HS (147 ± 1 days) than in SOFaaBSA (145 ± 1 day, p < 0.05). Reasons for this birth weight and gestation length difference are unclear, but our data suggest that different culture conditions can produce embryos with differing morphology, apparent chemical composition, and rate of development, resulting in lambs with differing gestation length and birth weight.