Background
Levator ani muscle (LAM) and anal sphincter injuries are common after vaginal birth and are associated with pelvic organ prolapse and anal incontinence.

Aims
Our objective was to investigate long‐term association between delivery mode, LAM avulsion and obstetric anal sphincter injuries (OASIS) in women at least 20 years after their first birth.

Methods
All women recruited at ‘index birth’ of the Dunedin (New Zealand) arm of ProLong (PROlapse and incontinence LONG‐term research) Study, were invited to have translabial and transperineal ultrasound assessment of LAM and anal sphincters. Post‐processing analysis of imaging data was performed blinded against delivery data. Statistical analysis was performed using the χ2 test and results are expressed as odds ratios (OR).

Results
Of the initial 1250 participants, 196 women returned for examination. Mean age was 50.8 years with a mean body mass index of 27.6 and median parity was three. They were seen on average 23 years after their first delivery. Four data sets were unavailable and one declined ultrasound assessment, leaving 191 for analysis. LAM avulsion was diagnosed in 29 (15.2%), and 24 women (12.6%) had significant anal sphincter defect. LAM avulsion was associated with forceps delivery (OR 2.45, 95% CI 1.04–5.80, P = 0.041). Forceps conveyed a greater risk of OASIS (21%) compared to a spontaneous vaginal delivery (11%) but did not reach statistical significance.

Conclusions
Forceps delivery is associated with long‐term injurious effect on pelvic floor structures. Discussions of the long‐term negative impact of pelvic floor structures and their functions are necessary to achieve an informed consent toward an operative vaginal delivery.