Diagnostic ultrasound ( PDFDrive )

1052 PART IV Obstetric and Fetal Sonography
A
B
FIG. 30.3 Implantation of the Blastocyst Into Endometrium. Entire conceptus is approximately 0.1 mm at this stage. (A) Partially implanted
blastocyst at approximately 22 days. (B) Almost completely implanted blastocyst at about 23 days. (With permission from Moore KL. The developing
human: clinically oriented embryology. 10th ed. Philadelphia: Elsevier; 2016. 1 )
hysterectomy specimens with an early pregnancy in situ. Before
12 weeks, the intervillous space contains no blood, only clear
luid, and on histologic examination, the villous tissue is separated
from the maternal circulation by a continuous layer of trophoblastic
cells. Only ater the third month does the trophoblastic
shell become broken and the maternal circulation become
continuous with the intervillous space. Furthermore, at weeks
8 and 9 of gestation, the trophoblastic shell forms plugs within
the spiral arteries, allowing only iltered plasma to permeate the
placenta. 17 In two-thirds of abnormal pregnancies, the
trophoblastic shell is thinner and fragmented, and the trophoblastic
invasion of the spiral arteries is reduced or absent. 18
Vascularization of the placenta occurs at the beginning of
the ith week. Oh et al. 19 showed signiicant increases in sac size
from 5 weeks onward in normal intrauterine pregnancy (IUP)
versus pregnancy failure. he rationale for placental vascularization
was based on early work by Folkman, 20 who showed that
tumors can grow to a size of 3 mm being nourished only by
difusion. To exceed this size, cells must recruit host blood vessels,
or the cells at the center will receive inadequate nutrition.