Preterm birth, defined as birth before 37 weeks of gestation, affects 5 to 18% of pregnancies. It is the leading cause of neonatal death and the second cause of childhood death below the age of 5 years. Preterm delivery (birth before 37 weeks' gestation) is further delineated into very early preterm (before 32 weeks), early preterm (32 0/7 to 33 6/7 weeks), and late preterm (34 0/7 to 36 6/7 weeks). It is logical to divide PTD into different subcategories: live-borns vs. intrauterine fetal deaths, singleton pregnancies vs. multiple pregnancies and spontaneous PTD vs. iatrogenic PTD. About 80% of all the preterm infants are live born singletons. The majority of these deliveries are spontaneous, due to onset of contractions or to spontaneously ruptured membranes. Conversely, iatrogenic preterm deliveries are due to the physician's decision to induce labor for maternal or fetal medical reasons. However, since the terminology varies, it is crucial to use clear definitions in all circumstances where the different phenotypical terms are used.

Preterm delivery is the consequence of four main mechanisms: activation of the maternal-fetal placental interaction with the hypothalamic-pituitary-adrenal axis, inflammation in the amniochorionic-decidual tissue, decidual hemorrhage and pathological distention of the myometrium. Although in most cases preterm births occur idiopathically, fetal, uterine, and placental factors as well as maternal chronic diseases, can affect preterm birth. It is estimated that of all cases of PTB, only 8.7% and 1.7- 2.3% of women, respectively, have the risk factors of prior PTB or significant cervical shortening in the midtrimester and would benefit from prophylactic therapy. A number of risk factors for preterm birth are known, but this does not mean that recognizing one of them in a pregnant woman before the 37th week means that a preterm birth will certainly begin. Knowledge of the risk factors allows for more intensive observation of the pregnant woman and gives obstetricians insight into what should be avoide . There are many maternal or fetal characteristics that have been associated with preterm birth, including maternal demographic characteristics, nutritional status, pregnancy history, present pregnancy characteristics, psychological characteristics, adverse behaviours, infection, uterine contractions and cervical length, and biological and genetic markers. Proposed techniques for detecting at-risk pregnancies include risk factor-based scoring systems, home uterine activity monitoring, maternal serum chemistries, salivary estriol, cervicovaginal chemistries, and amniotic fluid analytes. These modalities are characterized by either inadequate screening efficiencies, invasiveness, expense, or lack of commercial availability. Moreover, their use does not demonstrate reduced spontaneous preterm birth rates. Cervical length (CL) measured by transvaginal sonography (TVS) has been shown to be an effective predictor of spontaneous preterm birth (PTB). This finding has been confirmed in singleton and multiple gestations, in women with or without risk factors for preterm birth, and in asymptomatic women as well as women with preterm labor (PTL) or preterm prelabor rupture of the membranes (PPROM). The anterioruterocervical angle (aUCA) is a novel transvaginal ultrasonographic marker that showed evidence of being useful as a screening tool for spontaneous PTB. It can be used as an alternative to cervical length (CL) as screening method for sPTB. It involves measurement of the angle between the anterior lower uterine segment (LUS) and cervix. A wide, or obtuse, anterior uterocervical angle (aUCA) lends a more direct, linear outlet of uterine contents onto the cervix. A narrower, or acute, UCA supports an anatomical geometry that would exert less direct force on the internal os, which may be protective from cervical deformation. A wide aUCA ≥95ᶿ and ≥105ᶿ detected during the 2nd trimester was associated with an increased risk for sPTB\<37 and \<34 weeks, respectively. However, the studies that provided this evidence were retrospective and their results were heterogeneous. Our objective is to evaluate whether the anterior UCA can predict risk of sPTB in a general population of singletons and to evaluate its performance for predicting sPTB relative to CL.