The distribution of the stem villi arteries and arterioles diameter was evaluated on histological sections from 6 normal (control) and 8 Intrauterine Growth Restricted (IUGR) placentas. No significant difference between the diameter distributions was found. This result can be due either to a similar pattern in vascularization or to the inadequacy of the diameter distribution in showing existing differences. Further investigation is needed to make a distinction between these two hypotheses. A multiparametric model is proposed, which predicts the diameter distribution that is expected when a few parameters (describing the vessel geometry and the tree branching structure) are varied in their normal range. The diameter distribution is shown to slightly change or not to change at all when important parameters, such as the vessel asymmetry, are changed. We conclude that the diameter distribution is insensitive to variation in certain placental structural parameters. A reliable analysis of the stem villi vasculature in normal and TUGR placentas cannot be based on the comparison of the respective diameter distributions alone.