Scientists from two Dutch hospitals analysed nearly 300,000 vaginal births which took place in 1994 and 1995.
They found that nearly 2% of the women suffered serious tears to the perineum - the area of skin between the vagina and the anus.
Women whose birth was aided by using a vacuum extractor, rather than forceps, were less likely to suffer these tears.
This involves cutting the perineum to increase the size of the birth canal, so that the baby is less likely to cause further tissue damage during birth.
Common problems
In up to 85% of women who suffer serious, or third degree, tears of the perineum during birth, there is proof of improper healing of the anal sphincter.
Up to 50% may have problems of involuntary loss of gas or faeces - even after repairs have apparently been carried out successfully.
Many women also report pain during sexual intercourse after anal sphincter damage - this is a sign that the vaginal tissue has not properly healed.
Third degree tears are more likely if the baby is heavy, if the birthing process is long, or if the woman is giving birth to her first child.
Inducing labour also appears to slightly increase the risk.
Lead researcher Dr Jan Willem de Leeuw, of Ikazia Hospital, Rotterdam, told BBC News Online: "We think that our findings may be very important for daily practice.
"The use of a mediolateral episiotomy (a cut in the tissue between vagina and anus, in the direction of one of the buttocks, away from the anus) where there is a serious risk of a spontaneous tear or signs of foetal distress, may reduce the risk for damage of the anal sphincter by 80%."
Dr John Grant, editor-in-chief of the British Journal of Obstetrics and Gynaecology, said: "Faecal incontinence can be a devastating consequence of childbirth, resulting in physical and psychological illness and social isolation.
"The challenge for obstetricians is the use they make of this information.
"It would be wrong to advocate a policy of episiotomy for all, for a third degree tear is an uncommon obstetric complication."