Ruminant models of prenatal growth restriction.

Intrauterine growth restriction (IUGR) is a significant health issue that not only affects infant mortality and morbidity, but may also predispose individuals to coronary heart disease, diabetes, hypertension and stroke as adults. The majority of IUGR pregnancies in humans are characterized by asymmetric fetal growth, resulting from inadequate nutrient transfer to the fetus. Furthermore, most of these pregnancies involve functional placental insufficiency, and may also show altered umbilical velocimetry. As the severity of IUGR increases, the fetus becomes increasingly hypoxic, hypoglycaemic and acidotic. In addition, placental transfer or utilization of some amino acids is known to be altered in IUGR pregnancies. Although a great deal has been learned from clinical studies of human IUGR, appropriate animal models are required to define completely the mechanisms involved in the development of IUGR. The pregnant sheep is a long-standing model for placental-fetal interactions, and fetal growth restriction can be induced in pregnant sheep by maternal nutrient restriction, maternal nutrient excess, administration of glucocorticoid, utero-placental embolization, carunclectomy and maternal hyperthermia. Although all of these sheep models are capable of inducing fetal growth restriction, the degree of restriction is variable. This review compares these sheep models of IUGR with the characteristics of human IUGR.