**1. Introduction**

Prematurity is defined as labor before the completion of 37 weeks of gestation is globally a major factor in infants' morbidity and mortality. It has been estimated that annually almost 15 million premature infants are born, consisting of about 11% of all the laborers globally. 85% of these cases are infants that have completed 32–36 weeks of gestation, 10% are infants that have completed 28–31 weeks of gestation and 5% are infants that are extremely premature which means the ones who do not have completed 28 weeks of gestation [1–4]. Prematurity seems not only to result in 1.000.000 deaths annually but also to be the leading cause of infants' death. The percentage of morbidity related to prematurity is also high, as huge as the number of financial resources that must be spent in this public health sector [1–4].

The most important risk factor of prematurity is the existence of an obstetric history of prematurity. Nonetheless, the prediction of premature birth is not an easy task due to its complexity. Various well-known risk factors for preterm birth in singleton

pregnancies have emerged from many studies. Among other risk factors of prematurity are also multiple pregnancies [1–4].

What is more, the ultrasonographic estimation of cervical length before 24 weeks of gestation in singleton pregnancies is implemented during the second trimester of pregnancy. Studies show the inverse relationship between cervical length and preterm delivery, and since cervical length ultrasound assessment is a safe, reliable, and repeatable method, its use has been established in the second trimester of pregnancy aiming the detection of pregnancies at increased risk for premature birth and to the implementation of secondary prevention practices, which will be discussed later in this text. It has been estimated that women in triplet pregnancy with a cervical length < 25 mm between 15 and 24 weeks of gestation have a 51% risk of preterm delivery before the 32nd week of gestation. While in women that are at low risk of preterm birth and with the estimated risk of preterm birth in this population to be 4% (singleton pregnancies, absence of history of preterm birth in previous pregnancy), a correlation appears to exist between preterm birth and cervical length, measured by ultrasound between 19th and 24th week of pregnancy. In these singleton low-risk pregnancies it has been found that a cervical length of 15 mm occupies the third percentile, having a sensitivity of 8.2% and a specificity of 99.7% in the prediction of preterm birth below 32 weeks of gestation, with a cervical length of 25 mm corresponding to the tenth percentile in pregnancies of 16–24 weeks of gestation (threshold below which the cervix is described as "short") [5–10].

Furthermore, the risk of preterm birth appears to be higher when the detection of short cervical length occurs earlier in pregnancy. In addition, it is particularly interesting that the presence of a short cervix may somehow be added to other risk factors for preterm birth. For example, the combination of obstetric history of preterm birth and short cervix has a relative risk for preterm birth of 3.3, higher than the one that appears in pregnancies in which there is only a short cervix. Thus, it has been suggested by various organizations, such as ACOG, for women with a history of preterm birth to have their cervical length measured from the 16th to 24th week of pregnancy, while the question of whether it is worth applying the same practice to low-risk women is still controversial. The latter view is based on various studies suggesting that only a small proportion of low-risk, based on obstetric history, pregnant women will be able to be identified by ultrasound assessment of their cervical length (from the 5% of the primiparous women that eventually will give birth prematurely, only 23.3% of them have a short cervix in mid-pregnancy ultrasound). Another study found that although women with a cervix <25 mm give birth prematurely more frequently than women with cervical length > 25 mm calculated at mid-pregnancy, the vast majority of women with premature labor (82%) do not show a short cervix in the second trimester of pregnancy. Given the association between this cervical length in the second trimester of pregnancy and the risk of preterm birth, various medical practices have been proposed and studied, which are also applied in everyday clinical practice to prevent premature birth. The use of progesterone, cervical pessaries, and cervical cerclage each individually or all in combination used to treat "short" cervix, especially in singleton pregnancies, has raised many questions about the usefulness of these practices as measures to treat premature birth and also about the effectiveness of the above-mentioned treatment options [10–15].

Progesterone, the most researched and well-known technique of preterm birth prevention, can be administered in singleton pregnancies with a history of preterm delivery between 16 and 36 weeks of gestation at a dosage of 250 mg once weekly (17-OHPC), 200–400 mg orally, or 100–200 mg vaginally daily [10–15]. Studies have

#### *Cervical Length and Perinatal Outcome DOI: http://dx.doi.org/10.5772/intechopen.113835*

shown that administering progesterone via intravaginal or intramuscular injections of 17-hydroxyprogesterone from the second trimester of pregnancy through the third significantly lowers the risk of preterm birth in women with "short" cervixes, whether or not they have a prior history of preterm birth. However, as far as high-risk women are concerned, there are studies, such as OPPTIMUM (multicenter, double-blind randomized study) that based on their data there is no statistically significant reduction in the risk of preterm delivery with intravaginal progesterone administration. Therefore, more evidence is expected in this field to demonstrate the benefit of progesterone to each group of women and the most effective route of its administration [10–15].

The aim of this study was to collect information on the deduction of results concerning the relationship between cervical length and perinatal outcome. Although cervical length during the second trimester of pregnancy can be a helpful tool for identifying women who have a high risk of preterm birth, the current study concentrated on the cervical length, estimated by ultrasound between 20 and 22 weeks of gestation, and the risk of preterm birth in singleton pregnancies. Moreover, the present study was not only limited to singleton low or high-risk pregnancies for preterm birth but also included data concerning the relationship between cervical length and risk of preterm birth in twin and triplet pregnancies.

What is more, this study tried to examine the importance of cervical length measurement in the first trimester of pregnancy in the estimation of preterm birth risk. Such a relationship could be really useful since this would allow earlier medical intervention for the prevention of preterm birth.

Also, this study presented the relationship between cervical length in the third trimester of pregnancy and perinatal outcome (time of spontaneous onset of labor, success rates of labor induction). Those all are matters that highly concern obstetricians nowadays [10–15].

Generally, the aim of this study was through the review of the literature, the recognition of the women who are at high risk for preterm birth, and the implementation of strategies that could eventually prevent preterm birth.
