Antepartum Two-Dimensional Sonographic Evaluation of Levator Ani Muscle Coactivation and Correlation with Labor Outcomes in Nulliparous Women at Term Gestation

BACKGROUND:Levator ani muscle (LAM) coactivation consists in the inappropriate contraction - i.e. shortening - of the LAM during pushing effort. The condition is reported to occur in up to 30% of pregnant women prior to active labor, and has been associated with increased duration of the second stage in the context of the so-called soft tissue dystocia. OBJECTIVE:This study aimed to evaluate the correlation between levator ani muscle coactivation and labor outcome in a cohort of nulliparous women with singleton pregnancy at term. STUDY DESIGN:This was a single-center prospective observational study that included nulliparous women with low-risk term pregnancy with a cephalic-presenting fetus who were enrolled between 39 and 40 weeks of gestation. During routine term obstetrical assessment, trained midwives performed transperineal ultrasound on the midsagittal plane to evaluate the anteroposterior diameter of the levator ani muscle. The minimal distance between the posteroinferior border of the pubic symphysis and the anterior border of the puborectalis muscle was evaluated at rest and at the acme of a pushing effort. Levator ani muscle coactivation was defined as a reduction in the anteroposterior diameter of the levator hiatus during the pushing efforts. The primary outcome was to compare the duration of the second stage of labor between coactivating and noncoactivating women and evaluate the occurrence of composite adverse labor outcome as defined by the occurrence of any of the following conditions: episiotomy, vacuum or cesarean delivery for second-stage dystocia, obstetrical anal sphincter injury, or severe postpartum hemorrhage. RESULTS:A total of 201 women were included, of whom 42 (20.9%) had levator ani muscle coactivation. Compared with women without levator ani muscle coactivation, those with levator ani muscle coactivation showed longer active second stage of labor duration (64±45 minutes vs 84±50, respectively; P=.02) and higher rates of composite adverse labor outcome (12.5% vs 31.0%, respectively; P<.01) and obstetrical anal sphincter injury (5.4% vs 0.0%, respectively; P<.01). Levator ani muscle coactivation only was associated with composite adverse labor outcomes (34.3% in women with levator ani muscle coactivation vs 18.1% in those without levator ani muscle coactivation; P<.01). CONCLUSION:Within a population of women with singleton term pregnancies at low risk, the antepartum demonstration of levator ani muscle coactivation at transperineal ultrasound was associated with an increased duration of the second stage of labor and a higher frequency of composite adverse labor outcomes, including obstetrical anal sphincter injury. VIDEO ABSTRACT.