Diagnostic ultrasound ( PDFDrive )

CHAPTER 44 Cervical Ultrasound and Preterm Birth 1501
TABLE 44.1 Prediction of Spontaneous
Preterm Birth Based on Gestational Age (GA)
at Cervical Length Measurement (<25 mm) a
GA at
Delivery
<20 Weeks 20-24 Weeks 24-28
Weeks
<32 weeks 4.1 (1.6-10.1) 4.19 (2.6-6.7) No data
<34 weeks 6.2 (3.2-12.0) 4.40 (3.5-5.4) 4.0 (3.1-5.2)
<37 weeks 8.7 (3.8-19.9) 25.6 (8.5-76.7) 3.1 (1.1-8.9)
a Data presented as odds ratio ±95th percentile conidence intervals.
Modiied from Honest H, Bachmann LM, Coomarasamy A, et al.
Accuracy of cervical transvaginal sonography in predicting preterm
birth: a systematic review. Ultrasound Obstet Gynecol.
2003;22(3):305-322. 35
Several studies have attempted to improve the predictive value
of TVS by combining with other factors, including obstetric
history and the concentration of fetal ibronectin in cervicovaginal
secretions, to create a risk assessment formula.
PREDICTION OF SPONTANEOUS
PRETERM BIRTH
Obstetric Factors
Celik et al. 36 presented a model to evaluate the ability of combinations
of maternal demographics (age, race, weight, height, smoking
status, history of cervical surgery, obstetric history) and cervical
length between 20 and 24 weeks’ gestation for prediction of
SPTB in about 59,000 women with singleton pregnancies. he
best prediction for SPTB was provided by cervical length alone,
which was improved by adding obstetric history but not maternal
characteristics. he estimated detection rates for extreme (<28
weeks), early (28-30 weeks), moderate (31-33 weeks), and mild
(34-36 weeks) PTB by combining obstetric history and cervical
length were 80.6%, 58.5%, 53.0%, and 28.6%, respectively, with
a 10% false-positive rate. hese data suggest that the combined
screening model has better predictive value than either factor
alone; similar indings in smaller-scale studies have been
reported. 37
Fetal ibronectin (FFN) is a glycoprotein that binds the
amniochorion to the decidua and is released into cervicovaginal
luid in response to inlammation or separation of amniochorion
from the decidua. Recent studies suggest that the combined use
of cervical length and FFN improves the diagnostic performance
of each test. 38,39 However, it has been shown that positive or
negative FFN is relevant only when the sonographic cervical
length is less than 30 mm. 40 Risk of SPTB remains low in women
with cervical length of 30 mm or more and in those with cervical
length of between 15 and 30 mm and negative FFN. 38,39,41
Cervical Funneling
Much controversy surrounds the utility of measuring the cervical
“funnel,” which is deined as the dilation of the internal os and
the herniation of the fetal membranes into the cervical canal by
more than 5 mm 42 (Figs. 44.11 and 44.12, Video 44.1). To et al. 43
reported that funneling of the internal os was present in 4% of
all pregnancies; the shorter the cervix was, the greater was the
likelihood of funneling, with 98% prevalence if the length was
less than 15 mm and only 1% if greater than 30 mm. he rate
of SPTB was increased in pregnancies demonstrating cervical
funneling. However, funneling did not provide any signiicant
contribution to the prediction of SPTB when combined with
cervical length. 44,45 As an isolated inding compared with cervical
length, the residual closed length measurements have a better
predictive value than funneling. In addition, the shape or size
of the funnel was not correlated to SPTB. As such, funneling is
best reported as a categorical variable (present or absent) and
best interpreted in the context of overall cervical length and
obstetrical history.
Rate of Cervical Change
In their meta-analysis, Honest et al. 35 correlated a single measurement
of cervical length at a single time point in gestation with
the risk of SPTB. However, the events of prematurity occur along
a continuum and likely develop over an as-yet undeined and
variable period. herefore progressive shortening of the cervix
may be more important than a single abnormal cervical length
measurement. A “short and shortening” cervical length may be
a more efective tool for SPTB prediction than a “short but stable”
cervical length. In a cohort of unselected patients, Naim et al. 46
demonstrated that if cervical length decreased on serial scans,
the odds ratio for SPTB increased 6.8-fold per unit of change
(one unit = decline of 10 mm per month). In women at increased
risk of PTB, Owen at el. 28 reported that serial measurements up
to 24 weeks’ gestation signiicantly improved the prediction of
SPTB compared with a single cervical measurement at 16 to 18
weeks. Similarly, in women at increased risk of SPTB (based on
obstetric history), if cervical length remained stable from 12 to
20 weeks and greater than 25 mm, all patients delivered at term. 47
Patients in whom cervical length became serially shorter (ultimately
<15 mm) and the decrease in length occurred before 20
weeks were ofered and accepted a cervical cerclage, and all
delivered ater 30 weeks’ gestation. Groom et al. 48 and Szychowski
et al 49 reported similar results. hese studies suggest three
important points:
1. Serial cervical assessment is important to predict SPTB.
2. Progressive cervical shortening in patients at increased risk
may begin before the typical timing of routine cervical assessment
at the second-trimester fetal anatomic scan, advocating
for initiation of serial cervical assessment early in the second
trimester.
3. Serial cervical shortening in the second trimester may identify
patients with true mechanical failure of the cervix, who may
beneit from the placement of a cerclage to prevent SPTB.
Dynamic Cervical Change
A dynamic cervix is deined as having spontaneous shortening,
lengthening, or funneling observed during real-time TVS 50 (Fig.
44.13). However, the value of a dynamic cervix for the prediction
of SPTB is less clearly deined than that of a short cervix. Owen
et al. 28 demonstrated that dynamic cervical length shortening in