**3. Results and discussion**

Preterm birth, or the delivery of infants before 37 weeks of gestation, is a major contributor to neonatal morbidity and mortality and is therefore a problem with socioeconomic implications. It has been estimated that about 41,000 premature babies are born every day worldwide. Morbidity and mortality rates resulting from preterm birth are higher for "early" preterm infants (<28 weeks) and moderate preterm infants (28–32 weeks) and lower for "late" preterm infants (33–36 weeks). Since prematurity is associated with the appearance of various short- and long-term complications in neonates, such as retinopathy, cerebral hemorrhage, necrotizing enterocolitis, respiratory distress syndrome, neurodevelopmental disorders, one can easily understand the dimensions of this medical topic of prematurity [16–20].

Mortality rates appear to reach 44% for neonates 23 weeks old, 32% for 24 weeks old neonates, 12% for 25 weeks old neonates, 11% for 26 weeks old neonates, 8% for 27 weeks old neonates, and 2% for 28 weeks old neonates. The causes of death in preterm infants can be sometimes not clear and in each case, it is unclear which specific factor ultimately led to premature death. Often, neonates that end up in intensive care units for premature neonates have no other recognized cause of death than that of the general title of preterm delivery [16–20].

In Canada, these percentages have been translated into real costs and money that have to be spent in this sector of health. Specifically, the two-year survival rate was found to be 56% for "early" preterm infants, 92.8% for "moderate" preterm infants, and 98.4% for "late" preterm infants. Also, the mean hospitalization time for moderately preterm infants varied from 1.6 days at the age of 2 years to 0.09 days at the age of 10 years. The cost per neonate for the first 10 years of their life resulting from the above data is estimated to be \$ 67,467 for an "early" preterm neonate, \$ 52,769 for a moderate preterm neonate, and \$ 10,010 for a "late" preterm neonate. So, the national amounts, if we take into consideration the above costs, would be \$ 123.3 million for "early" preterm infants, \$ 255.6 million for moderately preterm infants, \$ 208.2 million for "late" preterm neonates, and \$ 587.1 million for all the newborns. So, based on the above medical and financial data, the importance of finding and adopting early prevention programs is fully understandable [16–20].

This can theoretically be achieved through the application of programs of cervical length measurement in the second trimester of pregnancy and its association with the risk of preterm labor. Such programs aim to identify and characterize women as high risk, in which "short" cervical length in combination with other characteristics from women's obstetric history will lead to the subsequent implementation of practices that will prevent preterm birth, leading to a significant reduction in prematurity rates.

Transvaginal ultrasound examinations of the cervical length have been shown to accurately predict preterm labor in both asymptomatic low-risk women and those who are already showing signs of preterm labor. Also, several studies have examined the use of transvaginal ultrasonography in high-risk asymptomatic women with significant differences to be noted between these studies regarding the cervical length that is considered as pathological, pregnancy age at which the measurement should be performed, and the gestational age that preterm labor eventually happens.

Several meta-analyses have examined the association between transvaginally measured cervical length and the prediction of preterm birth in asymptomatic women with singleton or multiple pregnancies. A systematic review of Crane, which included a sample of 2258 high-risk asymptomatic women with singleton

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

pregnancies, concluded that there was an inverse relationship between cervical length and the possibility of preterm labor. The cervical threshold under which the cervix was considered to be short was set at 25 mm, predicting with high sensitivity the risk of premature birth below 35 weeks of gestation, while women with a prior history of preterm labor and cervical surgery were identified as high risk for preterm labor. It was also found that the likelihood of preterm birth below 35 weeks is less in cases where cervical length < 25 mm is measured at 20–24 weeks of gestation, than the likelihood of preterm labor in cases where cervical length < 25 mm is measured at a gestational age less than 20 weeks or more than 24 weeks [20–24].

The women with a history of automatic onset of preterm birth included in the above analysis did not differ from those women who were getting into premature labor after premature rupture of fetal membranes and automatic onset of rhythmic contractions. And since the pathogenetic mechanisms in these two broad categories of preterm birth differ significantly, the predictive value of transvaginally measured cervical length was examined separately in these two subgroups. Some of the studies used in this analysis included a variety of risk factors for preterm birth, such as maternal age and smoking. These risk factors did not appear to have the same predictive value as other risk factors, such as history of previous preterm birth and presence of abnormalities in the uterus, so a subsequent statistical analysis was performed in this study and found that the results of the analysis were still the same even after exclusion of Watson's study that included smoking in older women in risk factors [24–28].

Previous studies have also noted the association between cervical surgeries and preterm labor. In Crane's meta-analysis, however, it was found that in women with a history of cervical surgery, cervical length less than 25 mm measured below 24 weeks of gestation was predictive of preterm birth before 35 weeks of gestation. Cervical length measured by transvaginal ultrasound at 24 weeks of gestation was found to be correlated with preterm labor before 37 weeks of gestation, but not with preterm labor before 34 weeks of gestation. In addition, this study showed that a cervical length less than 25 mm may be used as a predictive tool for preterm birth in women who have undergone correction surgery for uterine abnormalities. Last but not least, it was found that the most frequently used gestational age for cervical length ultrasound assessment and prediction of preterm birth is 20–24 weeks [28–32].

In a systematic review study, which included a sample of 1593 women and aimed to assess the accuracy of transvaginally measured cervical length during the second trimester of pregnancy in the prediction of preterm birth in twin pregnancies, it was found that this method of transvaginally measured cervical length works as a better predictor of preterm birth in twin pregnancies than in singleton pregnancies. For this reason, based on the analysis of Honest, women with singleton pregnancies and cervical length at 22–24 weeks of gestation less than or equal to 25 mm exhibited sensitivity and specificity in predicting preterm delivery below 32 and 34 weeks of pregnancy 4.2 and 0.4 and 4.4 and 0.67, respectively, while asymptomatic women with twin pregnancies and cervical length at 20–24 weeks of gestation below 20 mm exhibited sensitivity and specificity in predicting preterm delivery below 32 and 34 weeks of gestation 10.1 and 0.64 and 9.0 and 0.64, respectively [28–32].

Although the present study has, among other things, highlighted the value of transvaginally measured cervical length in the prediction of preterm birth in twin pregnancies, the efficacy of therapeutical practices that are based on the results of such studies and that are aiming to prevent premature birth is not certain. It is critical to note at this point that there is currently no therapeutic method that effectively

seems to prevent premature birth in twin pregnancies, as it has been suggested in singleton pregnancies the implementation of practices such as cervical cerclage and progesterone.
