Background: Vaginal delivery is the most important risk factors for development of faecal incontinence, which significantly affects quality of life. Foreign studies show OASIS occur at 20 to 40 % of vaginal deliveries. In Slovenia we recognize sphincter injuries at 1.7 % of deliveries, while true incidence of OASIS in our population remains unknown. Caesarean section prevents anal sphincter injuries. Known risk factors in foreign studies include prolonged second stage of labour, fetal weight > 3500 g, malpresentation, forceps delivery, maternal age more than 35 years at the time of first delivery, first delivery. Few women complain about defecatory problems in puerperium unless they are directly asked about them, so true incidence of such injuries is grossly underestimated. Previously compensated anal sphincter dysfunction can clinically manifest as late as in menopause. The most probable cause is atrophy of muscle and fibrous tissue of pelvic floor and anal sphincter due to lack of estrogen support in this period. With anal ultrasound we tried to determine the incidence of occult damage to anal sphincter in primiparas after vaginal delivery and the relation of injury to symptoms 6 weeks after delivery and identify possible risk factors in our population. We also tried to find out how many patients with anal sphincter injury become symptomatic immediately after deliv- ery. Methods: From January to June 2009 we examined 26 primiparas after vaginal delivery in the Ljubljana Maternity Hospital with anal ultrasound and compared various data about the delivery from our national delivery form. We excluded all patients with caesarean section, recognized anal sphincter injury at the time of the delivery or previous anorectal surgery, history of irritable bowel syndrome or pre-existing inflammatory bowel disease. All patients completed a bowel-function questionnaire, which included questions about faecal urgency and involuntary passing of gas, liquid or solid stools, before and six weeks after delivery. Faecal urgency was defined as inability to hold passing of stools for more than 5 minutes, anal incontinence as partial or complete inability to control passing of winds, liquid or formed stools. Patients were examined with 7 MHz 360-degrees rotating probe on the second or third day after delivery. With the probe we identified the U-shaped puborectalis muscle, then slowly extracted the probe through the anal canal towards the anus. We examined ultrasound im- age of puborectalis muscle, internal anal sphincter, longitudinal muscle and external anal sphincter. Internal anal sphincter (IAS) appears as a uniform hypoechoic circle, which is surrounded by heterogenous hyperechoic circle of external anal sphincter (EAS). External anal sphincter defect was defined as hypoechoic gap of various size in hyperechoic circle, that enlarges with voluntary contraction. Internal anal sphincter defect was defined as a gap in hypoechoic circle. All patients were contacted by telephone 6 weeks after delivery to complete the same ques- tionnaire again. Deliveries were managed by midwives according to standard active delivery management protocols the Ljubljana Maternity Hospital. All episiotomies were mediolateral. Information about pregnancy and delivery was obtained with patient’s consent from national delivery forms. We analysed use of analgesia at the delivery, induction and stimulation of labour, difference in body mass index (BMI) before pregnancy and before the delivery, duration of labour, fetal weight and head circumference and maternal age. Results: We found signs of external anal sphincter injury in 12 (46 %) out of 26 patients examined, all of them had only external sphincter injury. None of them had any de novo symptoms regarding defecation or problems restraining winds or stool 6 weeks after delivery (Table 1). There was no significant statistic difference for use of analgetics, stimulation of labour, vacuum extraction and episiotomy. There might be a difference in maternal age, but data was insufficient due to small number of patients. Conclusions: With our research we showed that incidence of sphincter injuries at vaginal delivery in our hospital is underestimated, as we found occult anal sphincter injury in 12 out of 26 patients. The number of patients was small so we were not able to estimate the importance of various possible risk factors for OASIS. All analysed cases showed no significant statistic difference due to small number of patients in the study. All patients were asymptomatic 6 weeks after delivery, but how many of them develop symptoms in later life remains unknown.