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a Chapter 17 Doppler Velocimetry of the Uteroplacental Circulation During Early Pregnancy 267
Fig. 17.17. Changes in diameter of the
main uterine artery at various gestational
ages. (Reprinted from [14] with permission)
Fig. 17.18. Changes in blood flow in the
uterine arteries (UA) during the first and
second trimesters of pregnancy
flow per week declined to 14 ml/min/week between
36±38 weeks' gestation [86].
During the first and early second trimesters the
decline in resistance to flow occurs not only in the
main uterine artery [14, 86] but also in the arcuate
[87], radial [88], spiral [88], and trophoblastic [87,
89] vessels. Trophoblastic Doppler signals are obtained
by placing the sample volume in the hyperechoic
area adjacent to the gestational sac. As could be
expected, the resistance to flow was progressively lower
as one moved from the main branch of the uterine
artery through the arcuate, radial, spiral, and trophoblastic
vessels. This phenomenon is demonstrated in
Fig. 17.19. When these measurements are performed
in a twin pregnancy, the resistance to flow is consistently
lower and volume flow rates are consistently
higher than those of singleton pregnancies (Tables
17.1 and 17.2, respectively). Values are presented as
the mean Ô SEM. These changes in the resistance to
flow during the early second trimester are consistent
with endovascular trophoblastic invasion of the decidual
portion of the spiral arteries, as previously described
[1±3].
Many investigators described a difference in blood
flow velocity between the left and right uterine arteries
during the first trimester [85, 90, 91] (Table
17.3). In the latter study [91] there appeared to be no
essential difference in frequency or magnitude be-