Uteroplacental circulation development: Doppler assessment and clinical importance

In the first weeks of pregnancy, columns of endovascular cytotrophoblastic plugs develop in the lumen of spiral arteries. Morphologic data show that these plugs become loosened as soon as the end of the second month and the intervillous circulation of maternal blood is likely to be established progressively between the 8th and 12th weeks. The disorganization of the musculo-elastic layers of these vessels provokes a dramatic decrease in vascular tone in the uteroplacental circulation. These modifications appear to govern the establishment of a low-pressure blood flow in the placenta, and hence determine the quality of uteroplacental circulation and normal fetal growth. Placental bed biopsies in women with pre-eclampsia and in a proportion of pregnancies with intrauterine growth retardation have shown that these physiologic changes were absent in the myometrial segments of spiral arteries. Recently, colour Doppler was used to assess intervillous and spiral artery flow in early pregnancy, confirming in vivo free intervillous flow at 12 weeks and a progressive significant decrease in spiral artery resistance with advancing gestation during the first trimester. However, certain data at an earlier gestational age are still contradictory. Particularly, the exact nature of the contents of the intervillous space before 8 weeks, and whether or not this fluid can be considered maternal blood, remains controversial.