Introduction
PTB, sPTB
PRETERM BIRTH, PRETERM DELIVERY

Multiple gestations are at higher risk for preterm birth (PTB), and neonatal morbidity and mortality, and their incidence has been in a rising trend since many of them are associated with assisted reproduction treatments.

The use of risk factors to identify women at higher risk of preterm delivery is part of the prevention strategies.

Studies involving singleton pregnancies have considered 25 mm as the most accepted cutoff to define a short cervix,
To correctly identify the CL that is associated to sPTB, specific populational distribution curves are necessary to describe the range of CL and to suggest what should be considered a short cervix in Brazilian twins' pregnancies. Moreover, it is important to know if it is possible to use CL as a predictor for sPTB in twin gestations. The main objective of this study was to describe a reference curve for CL in mid-trimester twin gestations using TVU and to identify the association between CL and gestational age at birth, and whether mid-pregnancy CL is a good predictor for PTB.