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Pediatr. SummaryPlacental blood flow in the maternal side and transfer of ["CIa-amino isobutyric acid (AIB) and lSHlmethylglucose (MG) were measured in 22 pregnant guinea pigs at various gestational ages. The fetuses were divided in three groups according to their body weights: small, average, and large. Body weight was 85.25% of average values in the small fetuses and 114.12% in the large fetuses. Placental weight was 121.73% of average in the large fetuses and 84.42% in the small fetuses. Placental blood flow was 134.48% of average in large fetuses and 73.18% in small fetuses. AIB and MC transfers were significantly lower in the small fetuses (80.33% and 86.06%, respectively, of average values). In contrast, in large fetuses, AIB transfer was 123.4356, and MC transfer was 113.96% of average. Significant correlations were found between fetal and placental weight and placental blood flow and transfer of AIB and MG in the various groups. Placental transfer of AIB and placental blood flow were significantly correlated in the small ( r = 0.59) and average weight fetuses ( r = 0.37). In addition, the slope of the regression cuwe for AIB was significantly steeper in the small fetuses when compared with the slope of average and large fetuses. Placental blood flow and transfer of MG were significantly correlated only in the large fetuses ( r = 0.48). In the small fetuses, the rate of AIB transfer was proportionally more reduced than that of MG transfer as the rate of placental blood flow decreased.The results demonstrate that spontmeous fetal growth retardation in the guinea pig is associated with a smaller placenta, a reduced placental blood flow, and a reduced transfer of AIB and MG. Inasmuch as in the small fetuses AIB transfer was proportionally more reduced than MG transfer, it is suggested that in addition to the reduced blood flow lMted availabwty of certain essential amino acids may be a cause of fetal growth retardation. SpeculationA reduced placental transfer of certain essential amino acids may further aggravate the fetal growth retardation associated with low placental blood flow.Fetal growth is influenced by the interaction of intrinsic or genetic factors and extrinsic factors imposed by the intrauterine environment. In turn, the intrauterine environment is influenced by such maternal factors as size, parity, smoking, and nutritional status (4). The mechanisms by which these maternal factors influence fetal growth remain unknown.In polytocous animals such as the mouse, the rat, and the guinea pig, the rate of placental blood flow is believed to be the major determinant of fetal growth. For instance, the inverse correlation between mean fetal weight and the number of fetuses in one uterine horn has been attributed to reduced placental blood perfusion (8). Similarly, the smaller size of the fetuses in certain positions within the uterine horns also has been attributed to hemodynamic factors (8). The relation of fetal growth to uterine blood flow is further supported by experiments demonstrating that a chr...

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