Diagnostic ultrasound ( PDFDrive )

1482 PART IV Obstetric and Fetal Sonography
A
B
FIG. 43.25 Bilobed Placenta. (A) Transabdominal sonogram of a third-trimester bilobed placenta. Both placental discs are of comparable size
(arrows). (B) Three-dimensional color Doppler ultrasound demonstrates the umbilical cord inserting into the upper lobe and the fetal vascular
connection between the lobes. See also Video 43.14.
he potential importance of the diameter of the umbilical
cord is unclear. In the irst trimester, fetal size correlates with
cord diameter, and small diameter may be a marker for pregnancy
loss. 131 Also, data from multiple centers suggest that cord diameter
may be a marker for chromosomal abnormalities when larger 132
or smaller than expected. 133 In the second and third trimesters,
the largest contributor to the size of the umbilical cord is Wharton
jelly, the supportive substance surrounding the umbilical vessels.
A nomogram has been developed for the area of Wharton jelly
that correlates with fetal biometry up to 32 weeks’ gestation. 134,135
In the second trimester, a larger-than-expected umbilical cord
is associated with aneuploidy. 136 IUGR has been associated with
thin cords; diabetes, fetal macrosomia, placental abruption, and
rhesus isoimmunization have been associated with thicker cords. 137
he associations between fetal umbilical cord size and fetal growth
overlap too greatly to be useful screening tools for these
entities. 138
Information on the umbilical cord and its manner of twisting
comes from the pathology literature. Let twists occur in 83%,
right twists in 12%, and absent twists in 5% of umbilical cords
in live-born singletons. For the umbilical cords that have a twist,
ascertainment of the degree of twist has been reported antenatally.
he umbilical coiling index is calculated by dividing the number
of helices by the cord length in centimeters (Fig. 43.26). he
mean umbilical coiling index is 0.44 ± 0.11 antenatally and 0.28
± 0.08 ater delivery. 139 Umbilical coiling does not vary with
respect to the amount of Wharton jelly present. 140 Assessment
of the degree of coiling in the second trimester does not correlate
well with the umbilical coiling index at term. 141
Absent umbilical cord twists are associated with single
umbilical arteries, multiple gestations, fetal demise, preterm
delivery, aneuploidy, and both marginal and velamentous umbilical
cord insertions 142-144 (Fig. 43.27). Lower degrees of coiling are
associated with lesser degrees of fetal growth. 145
True knots of the umbilical cord occur in 1% to 2% of
pregnancies. Although some are normal variants, 146 these knots
may also be associated with increased fetal mortality. Sonographic
A
FIG. 43.26 Umbilical Coiling Index. Deined as the distance (A)
between the same umbilical artery making one turn around the umbilical
vein. (With permission from Otsubo Y, Yoneyama Y, Suzuki S, et al.
Sonographic evaluation of umbilical cord insertion with umbilical coiling
index. J Clin Ultrasound. 1999;27[6]:341-344. 143 )
FIG. 43.27 Uncoiled Cord in Second Trimester.