الفهرس 
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Abstract
Preterm birth is defined as delivery before a gestational age of 37 weeks. It is one of the most vexing complications of pregnancy, affecting ” ” " ~ " ” ”11% of all births. It is the leading cause of neonatal mortality and a major cause of neonatal morbidity. Annually, more than 15 million children are affected worldwide. Mortality and morbidities of preterm labor include respiratory distress syndrome (RDS), intraventricular hemorrhage (IVH), necrotizing enterocolitis (NEC) and sepsis, are inversely associated with gestational age at birth. Furthermore, approximately one-third of neonatal deaths are caused by prematurity leading to more than 1 million newborn deaths per year (Wagner et al., 2015) Twin births are associated with excess morbidity and mortality for both mother and her children. The increased risk for adverse outcomes in twin gestations is largely attributed to preterm birth and the rates in twins are 5-6 times higher. The incidence of twin gestation has increased markedly over the past decades, mostly because of increased use of assisted reproductive technologies. They account for 17–20% of all preterm births, with about half of twin pregnancies delivering before 37 weeks of gestation (Iams et al., 2008) Sonographic measurement of cervical length (CL) by transvaginal sonography (TVS) has been shown to be an effective predictor of preterm birth. It has been found to be an efficient test for predicting preterm birth less than 34 week in asymptomatic women with risk of preterm birth. Cervical length below the 10th centile for gestational age is considered “short”. At 18 to 24 weeks’ gestation, the 10th centile corresponds to a cervical length of less than 25 mm (Hernandez-Andrade et al., 2012). Progesterone is a key hormone for pregnancy maintenance, and a decline in progesterone action is considered to be central to the initiation of parturition in most mammalian species, including primates (Lei et al., 2011).