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 hyster ectomy specimens contained within the Boyd Collection, ranging from 43 to 1 30 days' gestation. RESULTS: Before the eighth week of pregnancy, maternal arterial connections with the intervillous space are restricted to tortuous networks of interce llular spaces. Only after this period can direct channels be observed. Init ially, these are of small caliber, but they become sizable and clearly deli neated after 11 to 12 weeks. CONCLUSION: The maternal circulation to the placenta must be extremely slug gish 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 .