Objective: Description and assessment by 3-D transperineal ultrasound of modifications suffered by pelvic floor muscles during the passage of the fetal head through the birth canal during the second stage of labor, as well as the identification of the precise moment in which levator ani muscle avulsion takes place.
Materials and methods: Patients included were 35 primigravidae, recruited during the first stage of labor, with at term pregnancy (37-42 weeks), without serious maternal-fetal pathology and cephalic presentation. A prospective observational study of 35 primigravidae, recruited during the first stage of labor, with at term pregnancy (37-42 weeks), with fetus in cephalic presentation and without serious maternal-fetal pathology. Sonographic evaluation was carried out by 3-D transperineal ultrasound during the first and second stages of labor (with fetal head in 1st, 2nd-3rd and 4th planes of Hodge), immediately postpartum and 6 months postpartum. Ultrasound parameters studied were antero-posterior and transverse diameters, as well as levator hiatus area and levator ani muscle thickness and area.
Results: Twenty-one patients were studied (15 spontaneous deliveries; 6 instrumental deliveries). When measured with fetal head in the 4th plane of Hodge, a significant increase both in the levator hiatus area (15.39 cm2 /15.68 cm2 /20.96 cm2 /42.55 cm2 /22.92 cm2 /18.18 cm2 ; P < 0.0005) and in the levator ani muscle area (8.78 cm2 /9.18 cm2 /9.69 cm2 /15.07 cm2 /11.33 cm2 /12.36 cm2 ; P < 0.0005) was identified. Four cases of unilateral right avulsion (two vacuum and two forceps deliveries) were identified.
Conclusions: We conclude that the phase of delivery that causes a major increase in the area of the levator hiatus area and in the levator ani muscle area is when the fetal head reaches the 4th plane of Hodge. Furthermore, data in our paper indicates that the exact moment in which the avulsion of the levator ani muscle is produced is when the bulging of the fetal head on the maternal perineum occurs.
Keywords: labor; levator ani muscle; pelvic floor; transperineal 3D ultrasound.
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