Transvaginal sonographic screening of abnormal intra-amniotic structure around internal cervical os at 18-21+6 weeks of gestation

Objective: To detect an abnormal intra-amniotic structure around the internal cervical os using transvaginal sonography at 18-21(+6) weeks of gestation. Methods: In this cohort study, during a 12-month period from July 2022 to June 2023, 395 transvaginal scans were performed for second-trimester cervical-length screening in singleton pregnancies at 18-21(+6) weeks of gestation. Nine abnormal intra-amniotic structures (AIAS), 87 low-lying placentas (LLP, second-trimester placenta previa or placental margin within 2 cm from internal cervical os), and 299 controls (not LLP and no abnormal finding around internal cervical os) were diagnosed. Sonographic features of AIAS and delivery outcomes in the three groups were compared. Results: Sonographic features of nine AIAS cases were: two single bands, one single broad band, one multiple bands, one single tubular, and one multiple tubulars. Blood flow was noted in seven cases. Seven AIAS had disappeared by 26 weeks, one was resolved at 34 weeks, and one (vasa previa and parachute placenta) remained until delivery. Incidences of emergency cesarean section, abnormal placenta and umbilical cord, and abnormal deliveries in AIAS were significantly higher than those in LLP and control groups (P = 0.031). Blood losses during delivery in AIAS and LLP groups were significantly greater than in the control group (P = 0.004), but there was no significant difference in blood loss between AIAS and LLP groups. Conclusion: The incidence of AIAS around the internal cervical os in the second trimester was 2.28% (9/395 fetuses). Mid-second-trimester transvaginal sonographic screening for the detection of AIAS around the internal cervical os including vasa previa may be essential to reduce perinatal morbidity.