Labour is considered obstructed when the presenting part of the fetus cannot progress into
the birth canal, despite strong uterine contractions. It is more common in humans than in
primates, because the birth canal of a woman is not as straight and wide as in primates1.
The most frequent cause of obstructed labour is cephalo-pelvic disproportion - a mismatch
between the fetal head and the mother's pelvic brim. The fetus may be large in relation to
the maternal pelvic brim, such as the fetus of a diabetic woman, or the pelvis may be
contracted, which is more common when malnutrition is prevalent. Some other causes of
obstructed labour may be malpresentation or malposition of the fetus (shoulder, brow or
occipito-posterior positions). In rare cases, locked twins or pelvic tumours can cause
obstruction1.