**2. Method**

The search was made through Medline, Pubmed, EMBASE, and Cochrane Library using as keywords the following expressions: transvaginal ultrasound in pregnancy, transvaginal evaluation of cervix during pregnancy, mid-trimester evaluation of the cervical length, cervical length and prematurity, cervical assessment in twin pregnancy, cervical evaluation in triplet pregnancies, cervical length in the first trimester of pregnancy. The studies that were used in the present study examined the relationship between cervical length in the first, second, and third trimesters of singleton, twin, and triplet pregnancies and the risk of preterm birth.

The next step was to categorize all the relative studies that were gathered, based on some common characteristics of each subgroup study population. So, the following subcategories of studies were created: cervical length during the second trimester of singleton pregnancies and risk of prematurity. Cervical length during the first trimester-early second trimester of pregnancy and correlation with mid-trimester cervical length and risk of prematurity. Cervical length during the third trimester of pregnancy and prediction of spontaneous onset of labor, prediction of the risk of caesarian section, and success rates of labor induction. All the studies that were related to one of these subcategories were examined further.
