Color doppler assessment of the intervillous circulation in all trimesters of pregnancy

Abstract

Color Doppler sonography can be successfully used for the assessment of the intervillous circulation throughout the three trimesters of pregnancy. The first trimester pregnancies were assessed by the transvaginal route while second and third trimesters pregnancies were evaluated using transabdominal route. Color flow mapping was used for visualization of the outlets of the spiral arteries into the intervillous space and the blood flow inside it. Pulsatile, arterial-like intervillous flow signals and spiral arteries signals were assessed by the use of pulsed Doppler and expressed by resistance (RD and pulsatility (PD indices, peak systolic velocity (PSV), end-diastolic velocity (EDV), and temporal averaged maximum velocity (TAMV). Velocities of blood flow of pulsatile signal in the intervillous space increased significantly towards the mid-pregnancy.

Full Text

After reaching the plateau between 16 and 22 weeks of gestation they remained almost constant until the term. Near the term low-significant decrease of blood flow velocities were noted. The impedance to blood flow within the intervillous space expressed by Rl and PI is rather low. The impedance significantly decreases towards the mid-pregnancy and then remains stable. Continuous intervillous flow became stronger and randomly dispersed with advancing the gestation, while the mean peak systolic velocity reached the value of 27.4 cm/s before the delivery. It is our belief that the number of color-coded areas represents the formation of preferential blood flow pathways, while increment of blood flow velocities in the spiral arteries and intervillous space accompanied by decrement of the vascular impedance represents the degree of the trophoblastic invasion and lumen widening. During the early pregnancy there were only few color-coded areas where spiral and intervillous flow were detected. However, the vascular impedance measured at this site during early pregnancy was quite low and the blood velocities were relatively high. This fact suggests that from its establishment intervillous flow is substantial corresponding to the trophoblast invasion of spiral arteries. After reaching their maximum at mid-pregnancy, by the end of trophoblast invasion, blood flow velocities remain constant until the term. Around the term there is a slight, statistically low-significant, decrease of blood flow velocities. The impedance to flow in the intervillous space, measured in terms of resistance (Rl) and pulsatility (PI) indices, is low. It is characterized by significant decrease to the mid-pregnancy. Afterward it remains constant, and there is no significant change until the term. Low resistance is in concordance with the fact that the intervillous space is a voluminous vascular pool which enables the blood to flow on the villi that are freely floating inside it.