Objective To compare, in a prospective, randomised controlled trial, differences in anal sphincter function following forceps or vacuum assisted vaginal delivery in an institution practising standardised management of labour.

Design Prospective, randomised controlled trial.

Setting Tertiary-referral maternity teaching hospital.

Population One hundred and thirty women.

Methods Primiparous women were recruited antenatally and if an instrumental delivery was indicated, were randomised to either a vacuum or low-cavity, non-rotational forceps assisted delivery. Follow up consisted of a symptom questionnaire, anal manometry and endoanal ultrasound at three months postpartum.

Main outcome measures Faecal continence scores, anal manometry, endoanal ultrasound.

Results Sixty-one women delivered with forceps assistance (40 for failure to progress in the second stage) and 69 with vacuum assistance (33 for failure to progress); 16/69 vacuum deliveries proceeded to a forceps assisted delivery (23%). There were no statistical differences in the antecedent antenatal factors between the two groups. A third degree perineal tear followed 10 (16%) forceps and 5 (7%) vacuum deliveries. Based on intention-to-treat analysis, 36 (59%) women complained of altered faecal continence after forceps delivery compared with 23 (33%) following vacuum delivery three months postpartum (RR 2.88, 95% CI 1.41–5.88). Endoanal ultrasound was reported as abnormal following 34 (56%) forceps deliveries and 34 (49%) vacuum deliveries (RR 1.3, 95% CI 0.65–258). After exclusion of ‘failed vacuum’, median anal canal resting pressure was significantly lower following forceps delivery compared with vacuum delivery alone (P= 0.004). There were no significant differences in degree of ultrasound abnormality between the two groups.

Conclusions Symptoms of altered faecal continence are significantly more common following forceps assisted vaginal delivery. Based on continence outcome, when circumstances allow, vacuum should be the instrument of first choice in assisted delivery.