Poor maternal nutrition during gestation results in long-term postnatal changes in muscle morphometrics, muscle mass and fiber number, and intramuscular adipose deposition of the offspring. Thus, we hypothesized that under- and over-feeding ewes during gestation would inhibit fetal muscle development. To test this hypothesis, 82 pregnant Western Whiteface ewes were individually housed and fed 100%, 60%, or 140% of NRC requirements for TDN beginning at d 30.2 ± 0.2 of gestation. Offspring (CON, RES, and OVER, respectively), were necropsied at d 45, 90, and 135 of gestation and within 24 h of birth. At d 45 of gestation (n = 7 ewes per diet), the triceps brachii (TB), semitendinosus (STN), and longissimus (LM) muscles were weighed. The LM were frozen and cryosectioned to determine muscle fiber cross sectional area (CSA) and the number of muscle fibers per μm². Muscle weight was expressed as percent of fetal weight and data were analyzed using the MIXED procedure of SAS, with maternal diet as the main effect. Fetal weight tended to be less in OVER compared with CON (P = 0.07; CON: 11.0 ± 0.6 g, RES: 10.1 ± 0.5 g, OVER: 9.2 ± 0.5 g). No differences in the weight of TB, STN, or LM due to maternal diet were detected (P ≥ 0.53). The CSA of LM (n = 6 fetuses per treatment [2 fetuses per ewe]) from OVER and RES were smaller compared with CON (P ≤ 0.008; CON: 287.1 ± 3.2 µm²; RES: 223.4 ± 2.9 µm²; OVER: 254.9 ± 3.2 µm²). However, the number of fibers per µm² of LM was not different due to maternal diet (P = 0.47). Smaller muscle fiber CSA could result from inadequate protein accretion, decreased fusion of myogenic precursor cells, or both. Similar fiber numbers suggest increased interfibrillar space, which may lead to increased connective tissue formation. In conclusion, poor maternal nutrition during early gestation alters the formation of muscle fibers as early as d45, potentially resulting in decreased muscle mass at birth.