Diagnostic ultrasound ( PDFDrive )

1378 PART IV Obstetric and Fetal Sonography
A
B
FIG. 40.1 Secondary Ossiication Centers in Fetus at 38 Weeks’ Gestational Age. (A) View of the femur and distal femoral ossiication
center (arrow). (B) View of distal femur and proximal tibia with distal femoral ossiication center (arrow) and proximal tibial ossiication center
(arrowhead).
weeks. 17 In uncomplicated pregnancies the combination of a distal
femoral epiphysis of 3 mm or greater and the presence of a proximal
tibial epiphysis is considered a reliable marker of pulmonary
maturity. 18 Intrauterine growth restriction (IUGR) is associated
with a delay in ossiication of the distal femoral epiphysis and
proximal tibial epiphysis. he earliest secondary epiphysis to ossify
is the calcaneus, at approximately 20 weeks’ gestation, thus marking
the earliest point that assessment of delayed ossiication of the
secondary epiphyseal centers can be attempted.
he fascia within the muscle is highly echogenic compared
with the relatively hypoechoic cartilage. he fetal musculature
is slightly more echogenic than the relatively hypoechoic cartilage.
he fetal joint spaces, in particular the knee, appear echogenic
because of the combination of synovium, fat, and microvasculature.
13 he normal development and ultimate function of the
fetal musculoskeletal system depend on fetal movements, which
start by the second half of the irst trimester. In the absence of
normal fetal motion, the bones and muscles will be underdeveloped,
the chest will be narrow, and joint contractures and
postural deformities can occur.
Extremity Measurements
It is a standard practice to assess femur length (FL) as part of
the evaluation of fetal size and morphology. Although measurement
of all the long bones is not required in a routine obstetric
ultrasound, an overall evaluation of the fetal skeleton should be
performed to ensure the presence and bilateral symmetry of the
tubular bones. Available charts provide guidance for correlating
the length of the extremities with the gestational age (Table 40.3).
he longest femur measurement, excluding both proximal
and distal epiphyses, is usually chosen. he inclusion of the distal
femur point, or the specular relection of the lateral aspect of
the distal femoral epiphysis cartilage, is the most common reason
for overestimating FL 19 (Fig. 40.2A). An oblique FL measurement
will result in underestimate of length. he lateral border of the
femur in the near ield of the transducer appears straight, whereas
the medial border of the femur in the far ield has a curved
appearance 20 (Fig. 40.2B).
In the lower extremity the lateral bone is the ibula and the
medial bone is the tibia. he tibia and ibula end at the same
level distally. In the upper extremity, pronation may cause the
radius and ulna to cross, so it can be diicult to distinguish
the ulna from the radius using lateral and medial positions. he
ulna is distinguished from the radius by its longer proximal
extent and its relationship to the ith digit distally. he radius
and ulna end at the same level distally. Demonstration of this
relationship will efectively exclude the majority of radial ray
defects.
he clavicles grow in a linear fashion, approximately 1 mm
per week, and the gestational age in weeks is approximately the
length of the clavicle in millimeters from 14 weeks to term. By
40 weeks’ gestation, the clavicles measure approximately 40 mm. 21