The main target is to determine levator ani muscle avulsion rate in vacuum delivery, comparing it to forceps delivery. As secondary goals, The aim to evaluate the difference in levator hiatus area among our study groups.

Nulliparous women who were recruited for an initial evaluation from our maternity unit, Hospital Universitario Virgen de Valme. Participants were recruited prior to instrumentation at delivery and those meeting the inclusion criteria, being randomized into the two study groups (vacuum delivery or forceps delivery).

Deliveries completed using vacuum instrumentation were performed by obstetricians with a minimum of five years' experience in obstetric practice. In terms of analgesia, epidural analgesia was used for intrapartum analgesia. The forceps used for the instrumentation was the forceps of Kielland and the vacuum was a metal vacuum (Bird's cup 50 mm, 80 kPa) was used to perform fetal extraction. A suction cup was carefully placed over the flexion point, avoiding caput succedaneum, and rapid negative pressure was applied (over 2 min, until 0.6-0.8 kg/cm2 ). Traction was carried out during contraction, along with maternal push, at a rate of 2-3 tractions per contraction, and without associating Kristeller maneuver. The procedure was abandoned if, after three cup slides or 15 min, fetal extraction had not been successful. Selective episiotomy was carried out in VD following Valme's University Hospital clinical practice guideline for instrumental deliveries.

Obstetric parameters evaluated were: gestational age, labor induction, epidural analgesia, type of instrumentation, duration of second stage of labor, episiotomy and perineal tears. Fetal parameters studied after birth were: fetal sex, weight, head circumference, umbilical artery pH at birth, Apgar test result (at 1 and 5 min), presence of neonatal morbidity (cephalohaematoma, brachial plexus palsy, etc.), admission to neonatology department and neonatal mortality.

The sonographic evaluation was performed six months after delivery and was carried out by a single examiner, with more than five years experience exclusively in obstetric ultrasound, with specific training in 3/4D imaging and blinded to obstetric data relating to the delivery. A 500_ Toshiba Aplio (Toshiba Medical Systems Corp., Tokyo, Japan) ultrasound with an abdominal probe PVT-675MV 3D was used for the assessments. Images were acquired with patients in dorsal lithotomy position, placed on the gynecological examination table and under empty bladder conditions. The transducer was carefully placed on each patient's perineum, applying the minimal possible pressure. Three volume measurements were taken for each patient: at rest, with Valsalva maneuver and with maximum contraction. Then, offline analysis of ultrasound volumes was carried out. Analysis of ultrasound volumes was performed offline.

In the multi-view ultrasound images, complete avulsion was defined as an abnormal insertion of LAM in the lower pubic branch identified in all three central slices, i.e. in the plane of minimal hiatal dimensions (PMD) and the 2.5 and 5.0mm slices cranial to this one. Levator hiatus measurements, transverse diameters, anteroposterior diameters and area were also determined in the same plane (PMD).

Inclusion Criteria:

• Delivery with forceps or vacuum
• Cephalic presentation
• Primiparity
• At term gestation (37-42 weeks)
• No prior pelvic floor corrective surgery
• Written informed consent

Exclusion Criteria:

• Pregnancies with severe maternal or fetal pathology