Keywords
gestation - uterine cervix - biometry - morphology - transvaginal ultrasound
Descritores
Gestação - Colo uterino - Biometria - Morfologia - Ultrassom transvaginal
Introduction
Prematurity is a major cause of perinatal morbidity and mortality. A short uterine
cervix is a predictor of premature birth; therefore, its morphological and biometric
assessment is important.[1]
[2]
Cervical assessment is performed by clinical examination and ultrasound.[3] Bidigital examination requires dilation of the internal cervical os. It is, therefore,
a subjective assessment of the cervix, and it can underestimate the actual cervical
length by up to 12 mm.[4] The most accurate cervical assessment method is the transvaginal ultrasound, which
allows adequate observation of the internal and external cervical os and the endocervical
canal, with a biometric precision of ∼ 100%.[5] The morphological and biometric study of the uterine cervix should be performed
from the 14th week of gestation onward because the ultrasonographic differentiation between the
cervix and the lower uterine segment using ultrasound becomes difficult before this
gestational age.[6]
There is no consensus among authors regarding the value of cervical length that is
considered short and the associated risk for premature labor, the reported values
varying between 20 and 25 mm.[7]
[8]
[9] The importance of cervical biometry in screening patients for preterm labor has
been highlighted in several publications; however, studies that have investigated
the techniques of cervical measurement are rare. The evaluation of uterine cervical
morphology is important because it is technically more difficult to measure cervical
length in cases of curved uterine cervices, and the best measuring method in these
cases is still controversial.[10]
When the uterine cervix has a straight morphology, its measurement is performed in
a continuous manner, that is, along the cervical canal. When the cervix is curved,
the biometric measurement may be performed in two ways: (1) in a continuous manner,
for example, from the internal os to the external os of the cervix; and (2) in a segmented
manner, such as, by dividing the cervix into segments, with the measured cervical
length being the sum of these segments.[10]
[11]
Routine evaluation of uterine cervical length during the second trimester morphological
ultrasound is an important method to screen for preterm labor because most women do
not have risk factors.[12]
[13]
The objectives of the present study were to assess the cervical biometry between 18
and 24 weeks of gestation and to analyze the influence of uterine cervical morphology
(straight or curved) on the measuring technique for cervical length, for example,
segmented or continuous.
Methods
This was a prospective and cross-sectional study of transvaginal ultrasound assessment
of the uterine cervical biometry in pregnant women between 18 and 24 weeks of gestation.
The patients were recruited at a diagnostic imaging training center and at a fetal
medicine outpatient center, both located in the city of São Paulo, Brazil. This study
was approved by the Research Ethics Committee of the institution and a signed informed
consent was obtained from patients who agreed to participate voluntarily. The patients
received an explanation on the importance of assessing the uterine cervix and its
relationship with premature labor and associated complications.
The inclusion criteria were singleton pregnancies between 18 and 24 weeks of gestation,
with gestational age determined by the date of last menstruation and confirmed by
ultrasound performed up to the 13th week. The exclusion criteria were previous history of preterm labor; recurrent miscarriage
(two or more consecutive miscarriage); cervical manipulation, such as conization,
cervical amputation and cerclage; previous history of loop electrosurgical excision
procedure (LEEP)/large loop excision of the transformation zone (LLETZ); uterine and
fetal malformations, and increase or decrease of amniotic fluid index (AFI) (oligohydramnios
– AFI < 5 cm or polyhydramnios – AFI > 25 cm).
After undergoing the second semester morphological ultrasound, the pregnant women
were instructed to completely empty their bladders and remain in the gynecological
position to undergo transvaginal examination with an endocavity transducer of 5–6.5 MHz
and an opening angle greater than 120°. All the ultrasound examinations were performed
with a Logiq P5 apparatus (General Electric Healthcare, Milwaukee, WI, USA). The transducer
was covered with a lubricant and gel-free condom and was inserted into the anterior
vaginal fornix. This method allowed observing the morphology of the uterine cervix
(straight or curved), the internal and external os of the cervix, and the endocervical
canal surrounded by the cervical gland area. Continuous biometry was performed for
straight uterine cervices, in which the examiner placed one measuring caliper in the
internal os and another in the external os ([Fig. 1A]). For curved uterine cervices, the examiner used two measurement techniques; the
continuous technique as described above and the segmented technique, in which the
first measuring caliper was placed in the internal cervical os, the second caliper
was placed at the beginning of the cervical bend, and the last caliper was placed
in the external cervical os ([Fig. 1B]). The sum of these segments was described as the length of the uterine cervix. All
measurements were performed by only two experienced examiners.
Fig. 1 Biometry of the uterine cervix using the continuous (A) and the segmented (B) techniques.
The qualitative variables (gestational age, number of gestations, number of vaginal
deliveries, maternal age, ethnicity, smoking, curved cervix, and straight cervix)
were expressed as absolute and relative values, whereas the quantitative variables
(cervical length measured by both the continuous and the segmented techniques) were
expressed as means, standard deviations (SD), medians, and minimum and maximum values.
Polynomial regressions were performed to evaluate the correlation between cervical
length and gestational age, and the quality of fit was assessed using the coefficient
of determination (R2). The measurements of the uterine cervix performed using the continuous and segmented
techniques were compared using the paired Student t-test. The statistical analysis was performed using the SPSS software version 13.0
(SPSS Inc., Chicago, IL, USA), with the level of significance set at p < 0.05.
Results
The assessment included 752 pregnant women at gestational ages between 18 and 24 weeks,
with the following distribution: 87 (11.5%) at 18 weeks, 91 (12.1%) at 19 weeks, 86
(11.4%) at 20 weeks, 84 (11.2%) at 21 weeks, 83 (11.0%) at 22 weeks, 211 (28.1%) at
23 weeks, and 110 (14.6%) at 24 weeks. The mean maternal age was 30.3 ± 5.3 years
(16–42 years); 68.8% women were white, 51.2% were primigravidas, and 9.6% were smokers.
The results of the continuous cervical biometry varied from 38.7 ± 7.4 mm (16–60 mm)
at the 18th week to 36.3 ± 7.5 mm (12–53 mm) at the 24th week. The results of the segmented cervical biometry varied from 40.2 ± 7.8 mm (16–60
mm) at the 18th week to 38.1 ± 7.9 mm (12–53 mm) at the 24th week. Uterine cervical biometry did not vary significantly with gestational age and
was best represented by a linear equation both for the continuous (R2 = 0.0075) and the segmented (R2 = 0.0017) techniques ([Fig. 2]).
Fig. 2 Scatter plots of uterine cervical biometry, using the continuous (A) and the segmented (B) techniques, as a function of gestational age (B).
[Tables 1] and [2] show the percentiles 5, 10, 50, 90, and 95 of the cervical length, using the continuous
and segmented techniques as a function of gestational age, respectively.
Table 1
Percentiles of uterine cervical length (mm) using the continuous technique between
18 and 24 weeks of gestation
|
Percentiles
|
Gestational age (weeks)
|
|
18
|
19
|
20
|
21
|
22
|
23
|
24
|
|
5
|
26.0
|
26.0
|
31.3
|
30.0
|
26.4
|
27.5
|
22.0
|
|
10
|
29.0
|
28.4
|
33.0
|
32.0
|
29.1
|
31.7
|
27.0
|
|
50
|
39.0
|
38.0
|
39.0
|
40.0
|
38.3
|
38.7
|
37.3
|
|
90
|
47.2
|
46.8
|
49.0
|
47.5
|
45.3
|
46.7
|
44.9
|
|
95
|
52.6
|
53.8
|
56.3
|
48.7
|
46.2
|
47.9
|
46.8
|
|
n
|
87
|
91
|
86
|
84
|
83
|
211
|
110
|
Table 2
Percentiles of uterine cervical length (mm) using the segmented technique between
18 and 24 weeks
|
Percentiles
|
Gestational age (weeks)
|
|
18
|
19
|
20
|
21
|
22
|
23
|
24
|
|
5
|
26.0
|
26.0
|
31.3
|
30.0
|
27.2
|
31.0
|
23.1
|
|
10
|
30.0
|
30.0
|
33.7
|
34.0
|
30.0
|
33.0
|
28.0
|
|
50
|
41.0
|
39.0
|
40.0
|
42.0
|
40.0
|
41.0
|
39.0
|
|
90
|
50.0
|
48.0
|
50.0
|
49.0
|
47.0
|
49.8
|
47.9
|
|
95
|
54.4
|
53.8
|
56.2
|
51.7
|
48.8
|
51.0
|
49.0
|
|
n
|
87
|
91
|
86
|
84
|
83
|
211
|
110
|
[Table 3] shows that there was a statistically significant difference between cervical lengths
measured by both techniques at all gestational ages (p < 0.001), the mean difference being 1.7 mm ± 2.6 mm.
Table 3
Comparison between the continuous and the segmented techniques of cervical biometry
between 18 and 24 weeks
|
Cervical biometry
|
Gestational age (weeks)
|
|
18
|
19
|
20
|
21
|
22
|
23
|
24
|
|
Continuous (mm): mean (SD)
|
38.77
(7.45)
|
38.19
(7.88)
|
40.19
(6.73)
|
39.83
(5.78)
|
37.60
(6.87)
|
38.61
(6.24)
|
36.27
(7.56)
|
|
Segmented (mm): mean (SD)
|
40.20
(7.81)
|
39.51
(8.09)
|
41.02
(6.63)
|
41.63
(5.90)
|
39.00
(6.93)
|
40.82
(6.51)
|
38.12
(7.97)
|
|
Difference (segmented–continuous) mean (SD)
|
1.42
(2.36)
|
1.32
(2.30)
|
0.83
(2.15)
|
1.81
(2.69)
|
1.40
(2.92)
|
2.21
(2.50)
|
1.85
(3.37)
|
|
Paired Student t-test (p)
|
< 0.001*
|
< 0.001*
|
0.001*
|
< 0.001*
|
< 0.001*
|
< 0.001*
|
< 0.001*
|
Abbreviation: SD, standard deviation.
Chi-square test for trend: p = 0.001*.
Of these 752 pregnant women, 390 (51.9%) exhibited a curved uterine cervix and 362
(48.1%) had a straight cervix. [Table 4] shows the distribution of curved and straight cervical morphology between 18 and
24 weeks of gestation.
Table 4
Distribution of uterine cervical morphology between 18 and 24 weeks of gestation
|
Variables
|
Gestational age (weeks)
|
|
18
|
19
|
20
|
21
|
22
|
23
|
24
|
|
n (%)
|
n (%)
|
n (%)
|
n (%)
|
n (%)
|
n (%)
|
n (%)
|
|
Curved cervix
|
58 (66.7)
|
51 (56.0)
|
46 (53.5)
|
50 (59.5)
|
34 (41.0)
|
97 (46.0)
|
54 (49.1)
|
|
Straight cervix
|
29 (33.3)
|
40 (44.0)
|
40 (46.5)
|
34 (40.5)
|
49 (59.0)
|
114 (54.0)
|
56 (50.9)
|
|
Total
|
87
|
91
|
86
|
84
|
83
|
211
|
110
|
According to [Fig. 3], there was a predominance of curved uterine cervix up to the 21st week and a predominance of straight cervix after this gestational age.
Fig. 3 Proportions of straight and curved morphology of the cervical canal between 18 and
24 weeks of gestation.
Discussion
Until the late 1970s, the methods of assessing the uterine cervix consisted basically
of subjective methods, namely direct observation through speculum examination and
vaginal bidigital palpation. With the advent of the transvaginal ultrasound, in the
early 1980s, the morphology and biometry of the uterine cervix have been increasingly
studied, which led to a thorough investigations for the assessment of physiological
changes during pregnancy.[14]
In the present study, by means of transvaginal ultrasound, we determined reference
values for cervical length between 18 and 24 weeks of gestation; our sample was a
Brazilian population at low risk for preterm labor, with a small variation within
this gestational interval. Jafari-Dehkordi et al[15] determined reference values for cervical length between 8 and 38 weeks of gestation
using abdominal ultrasound in Iranian women. The mean cervical length did not show
much difference between 18 and 24 weeks of gestation (38.0 mm). A recent study established
conditional intervals of cervical length using transvaginal ultrasound between 11
and 40 weeks in 4,397 Greek women. They observed that the best correlation between
uterine cervical length and gestational age was a second-degree equation and the mean
length between 18 and 24 weeks changed from 32.1 to 31.3 mm.[16] In a recent Brazilian study, Peixoto et al[5] established a reference curve for cervical length in 996 singleton pregnancies between
20 and 24 weeks of gestation using the continuous technique of transvaginal ultrasound.
The values did not vary significantly with gestational age and the mean length was
37 mm in this gestational interval. In the randomized controlled trial, the mean cervical
length between 20 and 25 weeks of gestation was 33 mm. This value was lower than the
one found in our study (38 mm); however, there are several differences between both
studies, such as low versus unselected population different number of cases (752 versus
24,620) and type of pregnancy (singleton versus singleton and twin).[17]
With regard to morphology, there was a predominance of curved uterine cervices.[10]
[17] In the study by Yost et al,[18] to determine parameters that could be predictors of preterm labor between 16 and
18 weeks of gestation, the curved cervical morphology predominated over the straight
morphology (59% versus 41%); however, this type of morphology was shown to be a poor
predictor. The results of this study are in line with our findings, namely the results
of 51.9% of curved uterine cervix and 48.1% of straight cervix, thus confirming the
predominance of the curved morphology up to the 21st week. This morphology may be explained by local changes in collagen concentration
in the uterine cervix during the first trimester of gestation.[19]
We used two measuring techniques to determine the cervical length: continuous and
segmented. In women with a straight uterine cervix the measurements did not differ
significantly; however, cervical biometry is hindered when the continuous technique
is used in women with a curved cervix because the cervical length is often underestimated.
According to the literature, the segmented technique, when performed along the endocervical
canal, yields slightly higher yet more accurate values.[10] The most adequate method to measure the uterine cervix when there is a bend seems
to be the segmented technique; however, to prove this hypothesis it is necessary to
compare the measurements to a gold standard or to establish the correlation between
measurement by both techniques and the outcome of spontaneous preterm birth. So, in
the interval between the 18th and the 24th weeks of gestation, the ideal gestational age for measuring the curved uterine cervix
was the 21st week using transvaginal ultrasound. At this stage, the phenomenon of uterine conversion
has already occurred and the straight cervical morphology predominates; accordingly,
biometric measurements are easier to perform and are more accurate.
As limitation, neither intra- nor inter-observer reproducibility were performed. However,
all measurements were taken by only two experienced examiners, which could decrease
this reproducibility. Furthermore, all cervical length measurements were performed
by transvaginal route. In a previous study, transabdominal ultrasound measurement
overestimated the mean cervical length by 8 mm among pregnant women with a short cervix
and resulted in the underdiagnosis of 57% of cases.[20]
Conclusion
In conclusion, we determined reference values for uterine cervical length between
18 and 24 weeks of gestation in a Brazilian low-risk population using the continuous
and segmented techniques. The values did not vary significantly with gestational age.
The ideal gestational age for measuring cervical length was ≥21 weeks.
