ULTRASONOGRAPHIC MEASUREMENT OF LOWER UTERINE SEGMENT TO ASSESS RISK OF DEFECTS OF SCARRED UTERUS

Background Ultrasonography has been used to examine the scarred uterus in women who have had previous caesarean sections in an attempt to as sess the risk of rupture of the scar during subsequent labour. The pre dictive value of such measurements has not been adequately assessed, h owever. We aimed to evaluate the usefulness of sonographic measurement of the lower uterine segment before labour in predicting the risk of intrapartum uterine rupture. Methods In this prospective observational study, the obstetricians were not told the ultrasonographic findings and did not use them to make decisions about type of delivery. Eligibl e patients were those with previous caesarean sections booked for deli very at our hospital. 642 patients underwent ultrasound examination at 36-38 weeks' gestation, and were allocated to four groups according t o the thickness of the lower uterine segment. Ultrasonographic finding s were compared with those of physical examination at delivery. Findin gs The overall frequency of defective scars was 4.0% (15 ruptures, 10 dehiscences). The frequency of defects rose as the thickness of the lo wer uterine segment decreased: there were no defects among 278 women w ith measurements greater than 4.5 mm, three (2%) among 177 women with values of 3.6-4.5 mm, 14 (10%) among 136 women with values of 2.6-3.5 mm, and eight (16%) among 51 women with values of 1.6-2.5 mm. With a c utoff value of 3.5 mm, the sensitivity of ultrasonographic measurement was 88.0%, the specificity 73.2%, positive predictive value 11.8%, an d negative predictive value 99.3%. Interpretation Our results show tha t the risk of a defective scar is directly related to the degree of th inning of the lower uterine segment at around 37 weeks of pregnancy. T he high negative predictive value of the method may encourage obstetri cians in hospitals where routine repeat elective caesarean is the norm to offer a trial of labour to patients with a thickness value of 3.5 mm or greater.