Abstract Intrauterine feeding of the growth retarded fetus appears an attractive therapeutic possibility. However the factors which determine the reversibility of intrauterine growth retardation are poorly understood. While fetal substrate supply is the final common pathway by which many factors restrict fetal growth, improving fetal substrate supply does not always lead to improved fetal growth. Similarly, fetal substrate supply is an important regulator of fetal endocrine status, such as circulating IGF-1 levels, but again, improving fetal substrate supply does not always alter fetal endocrine status or fetal growth. The relationship between substrate supply, endocrine status and growth is regulated in a complex way by placental function. Understanding the role of the placenta in this regulation is essential if in the future we are to help the growth retarded fetus.