Cervical funneling or intra-amniotic debris and preterm birth in nulliparous women with midtrimester cervical length less than 30 mm - Saade, G R; Thom, E A; Grobman, W A; Iams, J D; Mercer, B M; Reddy, U M; Eunice Kennedy Shriver National Institute of Child Health and Human Development Maternal-Fetal Medicine Units (MFMU) Network (Ultrasound Obstet Gynecol, 2018)