The incidences, cooccurrences and epidemiological associations at term of the three common focal macroscopic placental lesions, infarcts, intervillous fibrin plaques (IVFP), and intervillous thrombi (IVT) were investigated as part of a population-based case-control study of small-for-gestational age (SGA) infants. Five hundred and nine placentas from women delivering SGA infants (10th percentile or less for gestational age) and 529 placentas from women delivering infants with birthweights appropriate for gestational age were examined using fixed protocols for identification of macroscopic lesions and microscopic diagnoses. One or more of these lesions were found in 280 placentas (28%), including infarcts in 150 (15%), IVFP in 132 (13%), and IVT in 64 (6%). Macroscopic misidentifications, particularly of IVFP as infarcts, emphasize the need for microscopic diagnoses. There were strong associations between the occurrence of any one type of lesion and cooccurrence of either of the other two, and these associations were site-dependent: between central (nonmarginal) infarcts and central IVFP (P = 0.0023); marginal infarcts and marginal IVFP (P < 0.0001); and between IVT (all central) and marginal infarcts (P < 0.0001) and marginal IVFP (P = 0.012). However, a study of associations between the incidences of placentas bearing each of the three lesions and 31 socio-demographic and pregnancy-related factors showed no associations in common. IVFP, an IVFP variant termed "labyrinthine," and IVT did not show any of the independent associations demonstrated between infarcts and SGA, pregnancy-induced hypertension, nonsmoking, age at first pregnancy, and ethnicity. IVFP had no significant associations, and IVT were associated only with male gender. The study has shown that IVFP and IVT do not share the important clinical associations demonstrated for infarcts, but has not identified the pathogenetic factor or factors responsible for the frequent cooccurrence of these lesions. The maternal thrombophilias may have such a role.