Disturbed trophoblast turnover, a key process in placental physiology, may lead to a number of pregnancy-associated pathologies. This study examines PCNA expression and describes and quantifies morphological changes during trophoblast turnover in preterm placentas and termplacentas complicated by IUGR. The number of CTF cells increased two-fold in preterm and IUGR placentas. A concurrent and slightly reduced proliferation rate of these cells was also found. The number of STF nuclei of terminal villi was lower by 21%in IUGR and by 18% in pretermplacentas (P>0.05). A statistically significant reduction of the number of syncytial knots by 50% as compared to the control placentas was observed. Correlations between PCNA-reactive CTF nuclei and syncytial knots, PCNA-reactive CTF nuclei and CCO activity, and CCO activity and syncytial knots were found. Moreover, a strong inverse relation was observed between syncytial knots and CTF cells, and CCO activity and CTF cells.