Maternal arterial connections to the placental intervillous space during the first trimester of human pregnancy: The Boyd Collection revisited

OBJECTIVE: We sought to determine morphologically when the maternal uterine circulation to the human placenta is established. STUDY DESIGN: We performed a histologic review of 12 early-pregnancy hysterectomy specimens contained within the Boyd Collection, ranging from 43 to 130 days' gestation. RESULTS: Before the eighth week of pregnancy, maternal arterial connections with the intervillous space are restricted to tortuous networks of intercellular spaces. Only after this period can direct channels be observed. Initially, these are of small caliber, but they become sizable and clearly delineated after 11 to 12 weeks. CONCLUSION: The maternal circulation to the placenta must be extremely sluggish before the eighth week of pregnancy but will be gradually established over the next few weeks. It is unlikely to be substantial until at least 12 weeks. The implication is that development of the human fetoplacental unit during most of the first trimester takes place in a low-oxygen environment.