Diagnostic ultrasound ( PDFDrive )

1500 PART IV Obstetric and Fetal Sonography
Relative risk of premature delivery
14
12
10
8
6
4
2
0
Relative
risk
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68
Length of cervix (mm)
No. of women
800
700
600
500
400
300
200
100
0
No. of women
1 5 10 25 50 75
Percentile
FIG. 44.9 Cervical Length and Risk of Preterm Delivery Percentile Ranking. Transvaginal ultrasound cervical length percentile rank, at 24
weeks’ gestation, and relative risk of preterm delivery before 35 weeks. (With permission from Iams JD, Goldenberg RL, Meis PJ, et al. The length
of the cervix and the risk of spontaneous premature delivery. National Institute of Child Health and Human Development Maternal Fetal Medicine
Unit Network. N Engl J Med. 1996;334[9]:567-572. 34 )
the 10th, 50th, and 90th percentiles of cervical length are 32.3,
41.9, and 50.5 mm, respectively. 33 A progressive linear reduction
in cervical length occurs over the 10th to 40th week of
gestation.
“Short” Cervix
With the goal of understanding the relationship between cervical
length and SPTB (delivery before 35 weeks’ gestation), in 1996,
Iams et al. 34 published a prospective, multicenter study in which
an unselected general population of women with singleton
pregnancies underwent TVS at 24 and 28 weeks’ gestation.
Cervical length at both examinations was comparable and
normally distributed, with a mean ±SD of 35.2 ± 8.3 mm at 24
weeks and 33.7 ± 8.5 mm at 28 weeks. A correlation between
cervical length and the rate of SPTB was determined (Fig. 44.9);
if the cervix was less than 26 mm (10th percentile) or less than
13 mm (1st percentile), risk of SPTB was increased by 6.49-fold
and 13.99-fold, respectively, compared with the rate of SPTB if
the cervix was at the 75th percentile length (40 mm) or greater. 34
Based on this landmark study, the deinition of a “short cervix”
as less than 25 mm (or <10th percentile length at 24-28 weeks)
was accepted (Fig. 44.10).
Since then, more than 50 studies of TVS evaluation of the
cervix and the risk of SPTB have been published. In 2003, Honest
et al. 35 conducted a meta-analysis of 46 studies (including more
than 31,000 asymptomatic singleton patients) and concluded
that the utility of TVS assessment of cervical length for the
prediction of SPTB varies with the gestational age at assessment
and the deinition of SPTB (birth before gestational age <32
weeks, <34 weeks, or <37 weeks). To summarize, the earlier in
gestation, the shorter the cervix, the greater is the risk of SPTB,
with the best predictive value when cervical length measures
less than 25 mm and SPTB is deined as delivery earlier than 34
weeks’ gestation (Table 44.1).
*
FIG. 44.10 Schematic of Abnormal Cervix. Length of the closed
cervical canal (L) and the presence or absence of funneling (*) should
be reported.
However, it should be recognized that the previous studies
assessed cervical length at or before 28 weeks’ gestation. Several
reports have demonstrated no predictive value of TVS measurement
of the cervix beyond 30 weeks of gestation for any deinition
of SPTB, likely because the cervix undergoes a gradual shortening
process beyond this gestational age regardless of the timing of
delivery.
he controversial aspect of TVS assessment of cervical length
and the prediction of SPTB is that not all patients with a deined
“short” cervix at any gestational age will deliver preterm. TVS
is a reasonable tool for identifying patients who will deliver at
or close to term with good negative predictive value. However,
more than 50% of patients with a cervical length less than 25 mm
measured at 20 weeks will deliver beyond 34 weeks’ gestation. 34,35
L