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GRAPEVINE, Texas -- When attempting a trial of labor after cesarean section, measuring the lower uterine segment thickness via ultrasound yielded no difference in the risk of maternal-fetal mortality and morbidity, a researcher said here.

There was no significant difference in risk for a composite outcome including maternal mortality, uterine rupture, stillbirth, and neonatal death among participants randomized to decision on mode of delivery based on lower uterine segment thickness, and those who attempted a trial of labor without it (RR 0.78, 95% CI 0.54-1.13), reported Philippe Deruelle, MD, of Hopital de Hautepierre in Strasbourg, France, and colleagues.

In an effort to avoid complications, such as uterine rupture or hypoxic encephalopathy, the rate of vaginal birth after C-section delivery has dramatically declined in both the U.S. and France, he reported at the Society for Maternal-Fetal Medicine annual meeting.

Deruelle pointed to prior research that showed lower uterine segment thickness was strongly correlated with intraoperative assessment, and that a thicker uterine wall meant a lower probability of a defect.

His group hypothesized that integrating ultrasonographic measurement of lower uterine segment thickness into the decision about the mode of delivery would result in lower maternal-fetal mortality and morbidity than usual care among pregnant women with a previous C-section delivery.

The was a randomized, unmasked trial conducted at eight centers in France. Each center had an expert sonographer trained by the principal investigator. Participants were age 18 or older, 36-38 weeks gestation with a live singleton fetus in a vertex presentation, and one previous low transverse C-section delivery.

If participants were randomized to the intervention, they were counseled based on their lower uterine segment thickness. Women with a lower uterine segment thickness greater than 3.5 mm were encouraged to attempt a trial of labor, while women with a lower uterine segment thickness 3.5 mm or lower were encouraged to try C-section delivery. The control group with no measurement all attempted a trial of labor.

There were 1,472 women in the intervention group -- 1,351 encouraged to attempt a trial of labor, and 84 encouraged to try C-section delivery, while 1,449 women in the control group had usual care.

Patients were a mean age of about 33, with a gestational age of 37 weeks, and more than half came from France.

Deruelle noted the mean BMI of 24 in each group, then quipped, that in the U.S., "it may be slightly different, sorry I had to say that."

Overall, there was a non-significant difference in risk between the intervention and control groups for the primary composite outcome (3.4% vs 4.3%, respectively), and no significant differences in risk of elements of the primary outcome.

There was also no difference in risk of secondary outcomes that included elective C-section delivery, C-section delivery during labor, and third and fourth degree perineal tears.

Deruelle also said that the observed rates of the primary outcome were lower than expected (4.1% vs 6.4%, respectively), and added that the study may have been "underpowered to demonstrate small differences."