Introduction: Levator ani muscle (LAM) lesions are the most frequent injuries of the pelvic floor during delivery. Ten to 36% of women report this lesion during their first delivery. Many risk factors have been proposed but very few evaluate the aspects that can influence during natural vaginal delivery.
Method: A prospective observational trial was conducted involving 74 primiparous women following vaginal delivery. Maternal, fetal and obstetric characteristics were analyzed. A transperineal three or four-dimensional (3D-4D) ultrasound was offered six months after delivery in order to evaluate avulsions and anomalies of the hiatus.
Results: Seventy four women were included, three of them did not show up for ultrasound evaluation. Sixty two (87.3%) demonstrated no avulsion in comparison with nine (12.7%) who did. Five of these lesions were unilateral and four bilateral. Mean newborn weight was 3193 g in the "no avulsion group" versus 3470 g in the "avulsion" group (p=0.025).
Discussion: According to the results, the most important risk factor established, for avulsion during natural childbirth, was the newborn weight. This contrasts with many other authors who have established that birth weight has no impact on these lesions.
Conclusions: The most important factor intervening in the avulsion of LAM during natural vaginal delivery is the newborn weight. Patients with diagnosed avulsions present an enlarged urogenital hiatus during valsalva and maximal contraction.
Keywords: Avulsion; childbirth; eutocic delivery; levator ani muscle.