Can Sonographic Evaluation of Lower Uterine Segment Predict Women at Risk of Uterine Rupture / Dehiscence?
Keywords:Lower uterine segment, uterine rupture, dehiscence, VBAC.
Introduction: Ultrasonography has been used to examine the scarred uterus in women who have had previous caesarean sections in an attempt to assess the risk of rupture of the scar during subsequent labour. This study aims to evaluate the usefulness of sonographic measurement of the lower uterine segment before labour in predicting the risk of intrapartum uterine rupture.
Methods: This is a prospective cohort study. Eligible parturients were those with one previous caesarean section who meet the inclusion criteria and were booked for delivery at Irrua Specialist Teaching Hospital. 153 patients underwent transvaginal ultrasound examination at 35-37 weeks' gestation, and were allocated to four groups (<2.5mm, 2.6-3.5mm, 3.6-4.5mm and >4.6mm) according to the thickness of the lower uterine segment. A systematic random sampling technique was used for patient selection. All labor was actively managed.
Inclusion criteria included women with 1 previous transverse lower uterine segment caesarean section scar presenting in spontaneous labor, singleton fetus with vertex presentation, non-recurrent indications for previous caesarean section e.g. malpresentation such as breech presentation, fetal distress and an estimated fetal weight (EFW) of less than or equal to 3.8 kg. The exclusion criteria included Women with a previous history of uterine rupture, women with fetal macrosomia, placenta previa, multiple gestation as well as abnormalities in amniotic fluid volumes such as polyhydramnious or oligohydramnious, women with co-existing medical conditions like hypertensive disease in pregnancy, uncontrolled diabetes mellitus in pregnancy, and women whose previous caesarean section was complicated by wound sepsis or wound breakdown.
Main Outcome Measure(s): The primary outcome in this study was the association between echographic measurements of the LUS and the risk of intrapartum uterine rupture. The secondary outcomes were trial of labor outcome (successful VBAC versus repeat Cesarean section), and determination of clinical (obstetric) factors that could serve as predictors for uterine rupture or dehiscence.
Results: The overall frequency of defective scar was 3.9% (2 ruptures, 4 dehiscences). The frequency of defects rose as the thickness of the lower uterine segment decreased: there were no defects among 49 women with measurements greater than 4.5 mm, 1 (1.4%) among 70 women with values of 3.6-4.5 mm, 2 (10%) among 20 women with values of 2.6-3.5 mm, and 3 (21.4%) among 14 women with values of 2.5 mm and below. With a cut-off value of 3.5 mm, the sensitivity of ultrasonographic measurement was 83.3%, the specificity was 80.3%, positive predictive value was 14.7%, and negative predictive value was 99.2% with an accuracy of 80.4%.
Conclusion: The results from this study showed that the risk of a defective scar is directly related to the degree of thinning of the lower uterine segment at around 37 weeks of pregnancy. The high negative predictive value of the study may encourage obstetricians to offer a trial of labour to patients with a thickness value of 3.5 mm or greater.
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