Routine 36-week scan: diagnosis and outcome of abnormal fetal presentation.

objectivesFirst, to report the incidence of non-cephalic presentation at a routine 36-week ultrasound scan, the uptake and success of external cephalic version (ECV) and the incidence of spontaneous rotation from non-cephalic to cephalic presentation. Second, to determine the maternal and pregnancy characteristics that provide a significant contribution to the prediction of non-cephalic presentation at the 36-week scan, successful ECV from non-cephalic to cephalic presentation and spontaneous rotation from non-cephalic to cephalic presentation.

methodsThis was a retrospective analysis of prospectively collected data from 107 875 women with a singleton pregnancy who had undergone a routine ultrasound scan at 35 + 0 to 36 + 6 weeks' gestation. Patients with breech or transverse/oblique presentation were divided into two groups: those scheduled for elective Cesarean section for a fetal or maternal indication other than abnormal presentation, and those that would potentially require ECV. The latter group was reassessed after 1-2 weeks and, if the abnormal presentation persisted, the parents were offered ECV or elective Cesarean section at 38-40 weeks' gestation. Multivariable logistic regression analysis was carried out to determine which maternal and pregnancy characteristics provided a significant contribution in the prediction of non-cephalic presentation at the 36-week scan, successful ECV from non-cephalic to cephalic presentation and spontaneous rotation from non-cephalic to cephalic presentation.

resultsAt the 36-week scan, fetal presentation was cephalic in 101 664 (94.2%) pregnancies and either breech, transverse or oblique in 6211 (5.8%). In 0.3% of cases with cephalic presentation at the 36-week scan, there was subsequent spontaneous rotation to non-cephalic presentation, and in half of these, the diagnosis was made during labor or at birth. ECV was attempted in 1584/6211 (25.5%) pregnancies with non-cephalic presentation at the 36-week scan and was successful in only 44.1% of cases. In the remaining 74.5% of cases, ECV was not attempted because of any of the following reasons: ECV was declined; Cesarean section was planned for a reason other than abnormal presentation; ECV was planned for the subsequent 1-2 weeks but, in the meantime, there was spontaneous rotation to cephalic presentation; or there was spontaneous onset of labor or rupture of membranes before planned ECV. In 5513/6211 (88.8%) pregnancies with non-cephalic presentation at the 36-week scan, ECV was not attempted or was unsuccessful, and in 37.7% of these, there was subsequent spontaneous rotation to cephalic presentation. Among the 6211 pregnancies with non-cephalic presentation at the 36-week scan, the presentation at birth was cephalic in 43.8%; in 74.8%, this was due to spontaneous rotation, and in 25.2%, it was due to successful ECV. Multivariable analysis demonstrated that the likelihood of non-cephalic presentation at the 36-week scan, that of successful ECV and that of spontaneous rotation from non-cephalic to cephalic presentation was affected by several maternal and pregnancy characteristics, but the predictive performance for these events was poor, with the area under the receiver-operating-characteristics curve ranging from 0.608 to 0.717 and the detection rate at a 10% false-positive rate ranging from 19.0% to 33.7%.

conclusionsRoutine ultrasound examination at 35 + 0 to 36 + 6 weeks' gestation could improve pregnancy outcome by substantially reducing the risk of unexpected abnormal presentation in labor. However, an additional ultrasound scan for fetal presentation should be considered in all women when they present in labor. © 2024 The Author(s). Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.