**2. Prevention of preterm birth**

According to the WHO, preterm birth is today the main cause of death in the first 5 years of life in the world. Thus, prevention of prematurity is of extreme importance to minimize the morbidity/mortality perinatal and the high costs involved with this disease.

Primary prevention begins with a good detection index of the problem. The previous preterm birth is undoubtedly a significant predictor of prematurity, and this is also valid for twin pregnancies [6]. The more premature the previous delivery was, the higher the risk of recurrence. In addition, other factors can contribute to increase preterm birth rates in multiple pregnancies, as race, schooling, smoking, and short cervix.

Some studies showed higher prematurity rates in black, younger, and low-schooling level pregnant women [13]. Smoking and primiparity seem to be related to shorter gestational age at birth too in twin pregnancy [8].

In secondary prevention of preterm birth, cervix evaluation is an important strategy, once uterine overdistension in multiple pregnancy can perhaps contribute to higher rates of short cervix and, therefore, higher rates of prematurity. So, many efforts were made to improve the prediction and prevention of preterm birth in twin pregnancy, in order to enhance the newborn prognosis, and cervix evaluation is one of them [14].

Transvaginal ultrasound for uterine cervix evaluation is currently the main tracking method for this severe disease [15], and second trimester ultrasound, between 18 and 24 weeks, is considered the best period to make the cervical transvaginal ultrasound.

A retrospective cohort study in twin pregnancy concludes that cervical shortening between 18 and 25 weeks of gestation was a good predictor of SPB [16].

In twins, the association of preterm birth frequently caused by uterine overdistension is largely aggravated by the presence of short cervix, and this association is more evidenced because of high indexes of preterm birth incompatible with extrauterine life.

It is true that a small performance improvement in this association of problems may completely change the prognostics of the newborn. Therefore, the recommendation of carrying out transvaginal ultrasound of the uterine cervix is of vital importance for diminishing preterm delivery in twin gestation.

In twin pregnancy, with the previous spontaneous preterm birth or late miscarriage but with atypical history of cervical insufficiency, strict follow-up of the uterine cervix is necessary from the 16th week of gestation, with transvaginal sonographic evaluation weekly, until the 24th week. If short cervical length (≤25 mm) is detected, a mechanical treatment (cerclage or cervical pessary) should be performed until 48 hours after diagnosis [7].

In the uterine cervix analysis, the evaluation of the cervical length ≤25 mm is considered the main predictor of preterm birth risk [10]. Also, other factors can be considered as preterm birth predictors, for instance, the presence of funneling signal [17], the presence of intraamniotic sludge [11], the absence of endocervical glandular echo (EGE) [18], and the presence of uterocervical angle >105° [19], as well as progressive diminishing of the cervix of more than 2 mm/week [15], must be considered also in twin pregnancy (Video https://mts.intechopen. com/download/index/process/279/authkey/236271ca370424655923c0bb7a7179a0).
