A preterm birth may be required in pregnant women who are at increased risk for preterm birth (e.g., multiple pregnancy, pregnant women with a previous history of preterm birth or cervical anomalies, pregnant women who have previously undergone a cervical surgery). Also, a preterm birth may be required after performing a transvaginal ultrasound to measure cervical length.
The aim of the cervical length measurement with 2 dimensional USG is to detect the increased risk for preterm birth before 34 weeks of pregnancy.
The risk for preterm birth increases as the cervix gets shorter.
A detailed anatomical examination of the first trimester is performed at 11-14 weeks in women at increased risk for preterm birth (e.g., multiple pregnancy, women with history of preterm birth, pregnant women with congenital structural uterine abnormality, and patients who have had cervical conization or leep). If this procedure is required then it should be repeated when a detailed USG is performed at 18-23 weeks.
A transvaginal ultrasonography is ideal to perform this type of ultrasound scan. Transperineal ultrasound may be used in cases where it is not possible to perform a transvaginal ultrasound.
Ideally, a short cervix refers to a cervical length equal to or less than 25 mm, which indicates the increased risk for preterm birth.
It is relieving to have a cervical length that is longer than 27 mm. But one should note that the cervix has a dynamic nature and can be shortened when contracting or in time. Therefore, a series of follow-ups may be required depending on the history and condition of the pregnant woman. The risk for preterm birth can be determined by measuring cervical volume with 3 dimensional USG or by obtaining a more ideal cervical length in the sagittal plane with a multiplanar mod.
For example, pregnant women with a cervical length greater than 15 mm at 23 weeks of pregnancy are known to deliver prior to 32 weeks of pregnancy with a rate higher than 50%.
Recently, many preterm births can be prevented by simply delivering progesterone support when identified a short cervix. A meta-analysis, published in 2013, did not find a significant difference between the outcomes from a cervical cerclage and the progesterone support to prevent preterm birth. Nevertheless, condition and characteristics of the pregnant women is important for the doctor to make this decision.
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