The environment to which the mammalian embryo is exposed during the preimplantation period of development has a profound effect on the physiology and viability of the conceptus (Gardner et al. 2005 Reprod. Fertil. Dev. 17, 361–370); therefore superovulation has been implicated in abnormal methylation and imprinting in the resultant embryo. In addition, the first week after mating is now known to be a critical time for embryo programming in sheep. However, the effect of altering the maternal environment by superovulation followed by embryo transfer (ET) into non-superovulated recipient ewes has not been thoroughly investigated in livestock animals. The objective of the present study was to determine if fetal and placental growth curves are affected by the mating type (AI or ET). In the ET group, 10 ewes underwent a superovulation program prior to artificial insemination; then 29 embryos were transferred singly into recipients. In the AI group, 25 ewes were inseminated with semen from the same sire as used for the ET group. All ewes used were Merino breed. At Day 30 after AI or Day 24 after ET, ewes were scanned for pregnancy using a 5-MHz transrectal probe (Honda, Tokyo, Japan). First, crown rump length (CRL, mm) was measured at Days 30 and 44, while abdominal circumference (ABD, mm) and femur length (FEM, mm) were measured monthly from Day 58 to term using a transabdominal 3-MHz convex probe. Data were analyzed by ANOVA for repeated measures. At Day 30, CRL was not different among AI and ET fetuses, but at Day 44, AI fetuses had significantly longer CRL than ET fetuses (P < 0.001, Table 1). Also, at the first measurement of FEM and ABD (Day 58) mating types were not significantly different, but measurements at Day 85 showed AI fetuses had larger FEM, P < 0.05, and ABD, P < 0.01. From Day 114 onward, there was no difference in fetal size between AI and ET fetuses. Univariate analysis of variance indicated no significant mating type or gender effects on gestation length, placenta weight, cotyledon count, weight of cotyledons as a proportion of total placenta, or weight of lambs at birth, prior to lamb-marking and weaning. We conclude that in the ET group, the first 90 days of the embryonic/fetal growth curve could subtly be affected by superovulation treatment; however, it did not affect weight and survival rate of this type of fetus.