CERVICAL LENGTH IN PREGNANCY

The length of the cervix is ​​the length between the internal os of the cervical canal and the external os. In pregnancy, the cervical length can be determined by ultrasound. The length of the cervix can be used to predict the risk of preterm labor that may occur in the next months of pregnancy, or to determine the high risk of birth in a patient diagnosed with preterm labor threat.
How long should the cervix be in pregnancy?
The length of the cervix remains relatively constant between 14-28 weeks, while it gradually gets shorter between 28-32 weeks.
It has been reported that cervical length is not affected by factors such as parity, race, and maternal weight.
Cervical length below 25 mm (2.5 cm) (10th percentile) is associated with the risk of preterm delivery. In a study, it was determined that 18 percent of the pregnant women whose cervical length measurements were made at the 24th week were below 25 mm, and 50 percent of those below the 13 mm (1st percentile) gave birth before 35 weeks.
The risk of preterm birth increases as the length of the cervix decreases. Especially when short cervical length is determined before the 24th gestational week, the risk is highest. The relationship between short cervical length and preterm birth is stronger before 24 weeks of gestation.
SCOG suggestions, one of the important guidelines published to date, are given below.
The Society of Obstetricians and Gynaecologists of Canada (SOGC)
- Transabdominal ultrasonography should not be used for cervical length assessment. Evaluation should be done by transvaginal ultrasonography. Transperineal ultrasonography should be preferred in patients who do not accept transvaginal ultrasonography.
- Routine cervical length assessment is not recommended in the low-risk patient group. (Some clinics scan the low-risk group between 18-24 weeks.)
- In the patient who is followed up with a diagnosis of preterm labor, cervical length measurement may be useful in distinguishing high-risk patients from others and reducing unnecessary treatment interventions. However, there is no clear evidence that this assessment reduces the rate of preterm birth.
- If there is funneling, the cervical length measurement should not include the funnel part. The part from the end of the funnel to the external osa should be measured.
- In patients with preterm labor, cervical cerclage can be performed in order to reduce the risk of preterm delivery if short cervical length is detected (less than 25 mm) before 24 weeks. In a meta-analysis, it has been reported that cerclage is not beneficial only in patients with short cervical length without previous preterm delivery, ie without additional risk factor.
- In patients presenting with a membrane that has been protruded up to the external cervical os, emergency cerclage should be applied to prevent preterm labor.
- More evidence is needed to recommend progesterone therapy in patients with short cervix.
How should cervical length measurement be done?
- (Prediction of Preterm Birth: Cervical Sonography, Seminars in perinatology)
- The patient's bladder must be empty when starting the measurement. Full bladder may mask cervical shortness.
- The transvaginal probe should be placed in the anterior fornix.
- A sagittal view should be obtained so that the entire endocervical canal can be observed.
- It is necessary to avoid pressing the probe too much because it can cause the cervix to lengthen. It should be noted that the front and back lips of the cervix are of the same thickness to understand that there is not much pressure.
- The image should be enlarged enough to cover at least 2/3 of the screen of the cervix.
- The length should be measured along the cervical canal from the internal os to the external osa.
- The shortest one should be recorded by making at least 3 measurements. Due to cervical or uterine contractions, a change in the length and shape of the cervix may be observed during measurement. For this reason, the measurement should be done for at least 5 minutes and the shortest measurement should be recorded.
Cervical length in patients diagnosed with preterm birth threats:
Measuring cervical length in this group of patients may be useful to differentiate patients at high risk of preterm labor. In this way, unnecessary tocolysis or other attempts can be reduced. In these patients, if the cervical length is over 30 mm, the risk of preterm delivery decreases significantly. When the cervical length is below 15-20 mm, the risk of preterm birth is high.


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