Objective: The effect of iron status in pregnant women on expression of ferritin receptor in placental microvilli membrane at mid-term gestation was investigated. Design: Ferritin receptor binding sites and dissociation constants (K d ) were determined in specimens of placental microvilli from 30 pregnant women at mid-term gestation and six women at term-delivery. Results: The ferritin receptor binding sites in the placental microvilli membrane in pregnant women with mild iron de®ciency and moderate iron de®ciency anemia were signi®cantly higher then those in pregnant women with normal iron status. However, no signi®cant difference was found between pregnant women with severe iron de®ciency anemia and with normal measurements. No signi®cant differences of K d values were detected between pregnant women with normal iron status and those with iron-de®ciency. Data also revealed that the ferritin receptor binding sites in placental microvilli membrane and K d values at mid-term gestation did not differ signi®cantly from those at term gestation. Conclusion: Lower iron status in pregnant women could lead to an increase in expression of ferritin receptor in placental microvilli membrane at mid-term gestation while the dissociation constant of ferritin receptor remains unchanged. This implies that the regulation of maternal ± fetal iron homeostasis via the ferritin receptor-mediated pathway is achieved by changes in the numbers of ferritin receptors rather then binding properties. Sponsorship: China Natural Science Foundation and The Hong Kong Polytechnic University. Descriptors: ferritin receptor; placental microvilli; iron status; pregnant women; maternal-fetal iron homeostasis