Keywords
cervical length measurement - fibronectin - pregnancy - preterm labor - risk
Palavras-chave
medida do comprimento cervical - fibronectina - gravidez - trabalho de parto prematuro
- risco
Introduction
Preterm delivery (PTD), defined as occurring before 37 weeks of gestational age (GA),
has an incidence of 11 to 18% of all pregnancies and is the main determinant of neonatal
morbidity and mortality.[1] This rate has remained constant in the past 50 years despite the various advances
in medicine, therefore indicating that primary and secondary prevention interventions
are occurring inadequately.[2]
[3] This rate remains at an average of 9.9% in Brazil.[4]
Preterm delivery is responsible for 75% of the cases of prematurity; the remaining
25% resulting from elective situations, such as preeclampsia, diabetes, fetal distress,
and others. The etiology of PTD includes numerous risk factors; however, its mechanism
remains uncertain.[5] Risk factors for PTD include a history of previous PTD, twinning, bleeding during
the second half of gestation, infections (chorioamnionitis, pyelonephritis, and asymptomatic
bacteriuria), black ethnicity, maternal age under 16 years or above 35 years, smoking,
chronic or acute maternal diseases, anemia, uterine malformations, trauma, placenta
previa, placental abruption, intrauterine growth retardation, and drug use. Nevertheless,
these risk factors occur in only 50% of all PTD cases, and they eventually contribute
to the identification of risk pregnancies.[4]
[5]
[6]
[7] The most important risk factor is a history of prior PTD.[8]
[9]
Recent studies demonstrate that the incidence of prematurity can be attenuated through
the use of the MUCL, evaluated at the end of the second trimester by transvaginal
ultrasonography (TVUS), to predict the probability of spontaneous PTD.[10]
[11]
Another useful test is the evaluation of the presence of fetal fibronectin (fFN) in
vaginal secretion. Fetal fibronectin is an adhesive glycoprotein produced by the trophoblast,
present in the maternal-fetal interface, which becomes detectable in the first half
of the pregnancy and after 35 weeks of GA. This protein will only be present in situations
of mechanical or inflammatory alterations due to damage to membranes or placenta between
22 and 35 weeks of GA. The detection of fFN in pregnant women with GA between 22 and
37 weeks can indicate the probability of evolution to spontaneous PTD; this measurement
has a high negative predictive value, therefore preventing unnecessary hospitalizations
and interventions.[4]
[6]
Current studies show that the use of the MUCL in combination with the detection of
fFN increases the sensitivity in predicting PTD. Iams et al[2] report very high rates of PTD recurrence (64%) in women with a positive fFN test
result and short MUCL (under 25 mm) in a multicenter study with 1,282 asymptomatic
pregnant women with a previous history of PTD. In that study, fFN appeared as the
most powerful predictor factor for PTD; in women with MUCL above 35 mm, the PTD recurrence
rate was 7% when fFN was negative compared with 28% in those with positive fFN test
result. These authors emphasize that the importance of these tests is greatest in
pregnant women presenting risk factors for PTD. In the case of pregnant women with
no history of previous PTD, the risk of birth before 35 weeks of GA was reported as
13% when the fFN test was positive and 8% when MUCL was shorter than 25 mm. Conversely,
when there was a history of previous PTD, the probability of a new event was 40% for
those with positive fFN and 30% for those with short MUCL.[12]
Hence, this study evaluated the risk of PTD through MUCL and fFN rapid-test results
in pregnant women hospitalized at Hospital Maternidade Santa Casa de Misericórdia
de Sobral (SCMS, in the Portuguese acronym), with complaints related to preterm labor
(PTL) prodromes.
Methods
This was an analytical, prospective, invasive, and non-interventional study, performed
at the SCMS with pregnant women presenting suggestive PTL symptoms, who were hospitalized
between September of 2015 and July of 2016. The sample size was calculated for convenience
and non-probability sampling.
The study was approved by the respective Ethics Committee in Research under protocol
number CAAE: 03996612.3.0000.5053.
The inclusion criteria were: pregnant women with PTL-related complaints, at GAs between
24 and 34 weeks, with cervix dilation ≤ 2 cm, and voluntarily acceptance to participate
in the study upon signing the free and informed consent term (all lengths of uterine
cervix were included).
The exclusion criteria were: patients with transvaginal bleeding, broken amniotic
sac, twin gestation, with reduced cognition/consciousness, and those who underwent
vaginal touch, used vaginal medications or douche, and had sexual intercourse in the
24 hours prior to study selection.
Initially, the endocervical material was collected from participants using only a
sterile swab; these samples were used to detect fibronectin through the fFN rapid
test. The presence of fibronectin in the cervical sample was determined qualitatively.
The vaginal touch would only be performed after the collection of endocervical material.
Subsequently, the MUCL was performed by TVUS. The following criteria were adopted
to ensure uniformity in this measurement: the internal cervical orifice should be
clearly visible with a gentle depression as an isosceles or funnel triangle; the entire
length of the cervical canal should be clearly visible; the external cervical orifice
should be visible symmetrically; the external cervix surface should be clearly identified;
the endocervical funnel, if present, was not included in the MUCL.
All cervix measurements were performed by the same examiner as well as the sample
collections for the fFN test. The cervix was determined as short when the length was
under 25 mm. Other data were collected through an interview based on a form with predefined
questions addressing socioeconomic, reproductive, and clinical-obstetric information.
The follow-up until delivery of outcomes on data annotation was conducted through
telephone contact with these pregnant women.
Data analysis was performed through the Epi-info software. The relative risk (RR)
was calculated for each parameter with 95% confidence interval (95% CI) and a significance
level of 5% (p < 0.05) in all tests. The risk assessment for PTD and testing for positive fFN, PTD
and MUCL, and risk assessment when both variables were positive were all evaluated
independently and included in the analysis. In addition, the risk for evolution to
childbirth was evaluated at 48 hours, 7 days, and 14 days when the fFN and MUCL tests
were each positive, separately or together.
Results
A total of 53 pregnant women were included in the analyses. The mean age was 22.80
years, 60.38% lived in urban areas, 52.83% had completed middle school education,
and 62.26% had a monthly income of up to 1 Brazilian minimum wage. The obstetric history
showed that 56.60% of the patients were primiparous. Out of the non-primiparous patients,
only 21.74% had a history of previous PTL. The mean GA at study admission was 31.83
weeks ([Table 1]).
Table 1
Characteristics of the analyzed sample
|
Average age (years)
|
22.8 (SD 7.32)
|
|
Origin
|
|
Urban
|
60.38%
|
|
Rural
|
39.62%
|
|
Family income (%)
|
|
Up to one minimum wage
|
62.26%
|
|
More than one minimum wage
|
37.74%
|
|
Education (%)
|
|
Illiterate
|
3.77%
|
|
Middle school level
|
52.83%
|
|
High school level
|
39.62%
|
|
College level
|
3.77%
|
|
Previous pregnancies (%)
|
|
Primiparous
|
56.60%
|
|
Non-primiparous
|
43.40%
|
|
Previous preterm delivery (%)
|
|
Yes
|
21.74%
|
|
No
|
78.26%
|
|
Gestational age at study admission (average weeks)
|
Average: 31.83 (SD 2.03)
|
|
Patients who received tocolysis (Nifedipine)
|
88.68%
|
|
MUCL by TVUTS
|
Average: 21.7mm (SD 8.9)
|
|
Cervix < 25mm
|
58.49%
|
|
Cervix ≥ 25mm
|
41.51%
|
|
fFN rapid test results
|
Positive
|
41.51%
|
|
Negative
|
58.49%
|
Abbreviations: fFN, fetal fibronectin; MUCL by TVUTS, measurement of uterine cervix
length by transvaginal ultrasonography; SD, standard deviation.
Almost all pregnant women received tocolysis when hospitalized (88.68%), but when
crossing that variable with preterm birth, we did not find a difference of risk (RR = 1.39,
95% CI, 0.46–4.21, p = 0.41).
The MUCL mean value measured by TVUS was 21.7 mm with a standard deviation of 0.89
and 75th percentile at 28.0 mm. The cut-off value of 25 mm used to transform this variable
into a qualitative one showed that 58.49% of the participants had MUCL shorter than
25 mm at the time of study admission.
A total of 41.51% of participants showed positive fFN rapid-test results during study
admission ([Table 1]).
Out of the 53 patients initially analyzed, 48 (90.57%) were followed-up until delivery;
of these, 26 delivered before 37 weeks (54.17%).
The comparison between time of delivery and MUCL showed a tendency of increased risk
of PTL when MUCL was shorter than 25 mm, however, without statistical significance
(RR: 1.83, 95% CI, 0.99–3.36, p = 0.09). Nevertheless, a statistical difference was observed between the meantime
in weeks from study admission to delivery (2.98 × 5.00, p = 0.03) ([Table 2]).
Table 2
Relation between measurement of the uterine cervix, fetal fibronectin rapid test results,
and the outcome of preterm delivery
|
MUCL < 25 mm X PTD
|
|
Relative risk
|
1.83
|
95% CI 0.99–3.36
|
p = 0.09a
|
|
Mean time between testing and delivery
|
|
MUCL < 25 mm
|
2.98 weeks
|
p = 0,03b
|
|
MUCL ≥ 25 mm
|
5.00 weeks
|
|
fFN rapid test X PTD
|
|
Relative risk
|
3.5
|
95% CI 1.39–8.79
|
p = 0.002a
|
|
Mean time between testing and delivery
|
|
Positive fFN
|
1.94 weeks
|
p = 0.0003b
|
|
Negative fFN
|
5.20 weeks
|
|
Both tests X PTD
|
|
Relative risk
|
2.70
|
95% CI 1.08–6.72
|
p = 0.002a
|
|
Mean time between testing and delivery
|
2.17 weeks
|
Abbreviations: CI, confidence interval; fFN, fetal fibronectin; MUCL, measurement
of uterine cervix length; PTD, preterm delivery.
p:p-value. Statical tests: a-X
2; b-T test.
The association of positive fFN rapid-test results with PTL was statistically significant
(RR: 3.50; 95% CI: 1.39–8.79; p = 0.002); the same occurred in relation to the mean time in weeks from study admission
to delivery (1.94 × 5.20, p = 0.0003) ([Table 2]).
The risk for PTL was also increased when both results (fFN and MUCL) were used; however,
this risk was lower than that considering the fFN rapid test alone (RR: 2.70; 95%
CI 1.08–6.72; p = 0.002) as demonstrated by the average number of weeks from study admission to delivery
(2.17) ([Table 2]).
The MUCL results showed that the relative risk of PTL is RR = 1.30, 95% CI 1.02–1.67,
and p = 0.11 in the first 48 hours; RR = 1.43, 95% CI % 0.99–2.06, and p = 0.12 in up to 7 days; and RR = 2.03, 95% CI 1.26–3.27, and p = 0.008 in up to 14 days ([Table 3]).
Table 3
Risk for preterm delivery outcome in 48 hours and 7 and 14 days relative to measurement
of the uterine cervix and fetal fibronectin results
|
Evolution to PTD
|
|
Within 48 hours
|
In up to 7 days
|
In up to 14 days
|
|
MUCL < 25 mm
|
F = 16.67%
|
F = 25.00%
|
F = 35.41%
|
|
RR = 1.30
|
RR = 1.43
|
RR = 2.03
|
|
p = 0.11
|
p = 0.12
|
p = 0.008
|
|
95% CI 1.02–1.67
|
95% CI % 0.99–2.06
|
95% CI 1.26–3.27
|
|
Positive fFN
|
F = 18.75%
|
F = 29.17%
|
F = 33.33%
|
|
RR = 1.75
|
RR = 2.88
|
RR = 3.57
|
|
p = 0.0006
|
p = 0.0001
|
p = 0.0002
|
|
95% CI 1.20–2.53
|
95% CI, 1.57–5.31
|
95% CI 1.63–7.81
|
|
MUCL < 25 mm
&
positive fFN
|
F = 16.67%
|
F = 22.91%
|
F = 27.08%
|
|
RR = 1.74
|
RR = 2.22
|
RR = 2.76
|
|
p = 0.0001
|
p = 0.0001
|
p = 0.0002
|
|
95% CI 1.14–2.64
|
95% CI 1.22–4.04
|
95% CI 1.27–5.96
|
Abbreviations: CI, confidence interval; F, frequency; fFN, fetal fibronectin; MUCL,
measurement of uterine cervix length; p, p value; PTD, preterm delivery; RR, relative risk.
Statistical test: X
2.
When the fFN rapid test was positive, the relative risk for PTL in 48 hours was RR = 1.75,
95% CI 1.20–2.53, and p = 0.006; RR = 2.88, 95% CI, 1.57–5.31, and p = 0.0001 in up to 7 days; and RR = 3.57, 95% CI 1.63–7.81, and p = 0.0002 in up to 14 days ([Table 3]).
A significant association with PTL was observed in all scenarios when evaluating the
positivity of both tests (MCCU < 25 mm and positive fFN); the relative risk for PTL
was RR = 1.74, 95% CI 1.14–2.64; and p = 0.0001 in 48 hours; RR = 2.22, 95% CI 1.22–4.04, p = 0.0001 in up to 7 days; and RR = 2.76, 95% CI 1.27–5.96, and p = 0.0002 in up to 14 days ([Table 3]).
Discussion
Although digital uterine cervix evaluation is part of the routine examination of patients
who are at high risk for prematurity, this method is not often a safe way to recognize
early cervical alterations. Yamasaki et al[13] verified that the MUCL by TVUS showed a better accuracy for PTL diagnosis than vaginal
touch in pregnant women at high risk because the portion above the anterior fornix
can be evaluated through TVUS but not through vaginal touch.
In this study, considering only the cervices < 25mm, there is no statistically significant
increase of risk of PTD, but the interval of time between the measurement and the
delivery was statistically different, two weeks longer in pregnant women when the
MUCL > 25mm. In 1996, Iams et al[14] reported that MUCL under 25 mm indicated a PTD positive predictive value of 17.8%
and negative of 97%. Therefore, the MUCL helps distinguishing pregnant women from
false PTL, which may prevent unnecessary interventions. Likewise, a pregnant woman
with short MUCL deserves extended attention through the performance of additional
tests using antenatal corticoid and preventive measures.
Tanvir et al (2014),[15] evaluated the MUCL in 130 pregnant women between 22 and 24 weeks of GA using the
length < 25 mm as the cut-off point. That study reports that among the patients with
short MUCL (16 women), 13 evolved to PTD, thus demonstrating the importance of this
measurement as a PTD predicting factor.[15] To et al (2001)[16] demonstrated the inverse relationship between PTD risk and MUCL; the risk can reach
78% when the length is shorter than 5 mm, decreases to 4% when the length is up to
15 mm, and decreases to 0.5% when the length is greater than 50 mm in asymptomatic
patients. Our study highlighted a similar trend; the difference in the magnitude of
the reduction may be due to the small sample, did not allow our results to reach statistical
significance.
Fetal fibronectin is an extracellular matrix glycoprotein that is produced by amniocytes
and cytotrophoblasts and has been shown to predict spontaneous preterm birth.[17] Our study observed the use of the qualitative fFN test showed an increase of PTD
isolated or in association with MUCL, and when the test was negative, the time between
to delivery was greater than three weeks, showing superior to MUCL. Similar results
in a multicenter study, Brujin et al (2016),[18] demonstrated the comparison between the quantitative and qualitative results of
fFN tests associated with MUCL and the risk for PTD in seven days. These authors showed
that the quantitative fFN test presents the same accuracy compared with the qualitative
fFN test associated with MUCL. However, the association between MUCL and the qualitative
measure of fFN shows an advantageous capacity to predict PTD.[18] Other studies demonstrated that the use of the qualitative fFN test alone was not
enough to increase the prediction of PTD compared with the use of the fFN test in
association with MUCL.[19]
[20] Deshpande et al (2013),[21] evaluated the cost-effectiveness of using the rapid fibronectin test in symptomatic
pregnant women and reported that the test, used in isolation, had moderate accuracy
and could identify patients with negative test results who would not need intervention.
Magro-Malosso et al (2017),[22] observed that positive fFN tests were detected in 33% of symptomatic patients and
significantly associated with PTD at 34 weeks and within 48 hours, 7, 14, and 21 days
after admission (p < 0.05); this association was not observed in the asymptomatic group. This study
showed an increased risk of PTD, mainly when analyzed the use of fFN test and both
tests. The MUCL increased the risk of PTD only within 14 weeks. Van Baaren et al (2013)[23] concluded that the best cost-benefit ratio is in the combined application of the
fFN test and MUCL. Hadži-Legal et al (2016)[24] reported similar results in symptomatic pregnant women observing that the combination
of these tests turned out to be an excellent predictor of PTD within 14 days of admission.[24] Despite the similarities to our results, these authors state that there are no high-quality
studies about the evaluation of this test, reducing the magnitude of our results.
In another study with a small sample of 30 patients, the fFN test used in this sample
was not shown to be predictive enough to inform the decisions of clinicians and pregnant
mothers to delay evacuation to a regional birthing center.[25] Similar results in systematic review and meta-analysis of randomized clinical trials
the fFN testing in singleton gestations with threatened preterm labor is not associated
with the prevention of preterm birth or improvement in perinatal outcome but is associated
with higher costs.[17] The different trends shown by our results may be due to the small sample and a distinct
population group of Brazilian women not included in these studies.
Our study has several strengths, the main one being that it is the first study in
the Northeast of Brazil that associates fFN e MUCL in PTD, and some weaknesses, including
the facts that this study is analytical and non-interventional and has a small sample.
Conclusion
In symptomatic pregnant women, we conclude that the MUCL < 25 mm and positive fFN
rapid test indicate increased risk for PTD. Further studies with larger sample sizes
could contribute in supporting the results presented in the current study.
