The effect of replacing vacuum with forceps in operative vaginal delivery: an observational study

An increase in vaginal delivery with forceps may increase rates of pelvic floor trauma. This study was designed to predict trauma rates resulting from policies preferencing forceps.

This is an observational cohort study utilizing data from 660 primiparae enrolled in an RCT in two tertiary obstetric units in Sydney, Australia. Participants were assessed clinically and with 4D translabial ultrasound in the late third trimester and again at 3-6 months postpartum. Incidence of trauma associated with mode of delivery was adjusted to reflect change associated with a conversion of vacuum to forceps delivery. Primary outcome measures were third-/fourth-degree tear, levator avulsion (LA) and external anal sphincter (EAS) trauma diagnosed sonographically.

Five hundred four women were seen at a mean of 5.1 (2.3-24.3) months postpartum. After exclusion of 21 because of missing data, 483 women were analysed: 112 (23%) had a CS, 268 (55%) a normal vaginal delivery (NVD), 69 (14%) a vacuum (VD) and 34 (7%) a forceps (FD). One hundred fifty-two women had EAS trauma and/or LA; 17 sustained both. After VD, 32/69 (46%) women suffered LA and/or EAS trauma; after FD, it was 33/34 (97%). Converting VD to FD was estimated to result in an increase in trauma from 152/483 (31%) to 187/483 (39%). A formula can be generated based on local obstetric and ultrasound data to estimate trauma incidence.

A change in obstetric practice resulting in the conversion of primary VD to primary FD would be expected to substantially increase the likelihood of pelvic floor trauma.