In the majority of cases there is no obvious reason why the fetus should present by the breech at full term. In practice, what is commonly observed is the association of breech presentation at delivery with a transverse lie earlier in the pregnancy, i.e. the fetus lies sideways across the mother's abdomen, facing a sideways implanted placenta. It is thought that when the placenta is in front of the baby's face, it may obstruct the normal process of inversion, when the baby turns head-down as it gets bigger during the pregnancy. As a result, the fetus turns in the other direction and ends in the breech presentation. Some other circumstances that are thought to favour a breech presentation during labour include:

• Premature labour, beginning before the baby undergoes spontanous inversion from breech to vertex presentation
• Multiple pregnancy, preventing the normal inversion of one or both babies
• Polyhydramnios: excessive amount of amniotic fluid, which makes it more difficult for the fetal head to 'engage' with the mother's cervix (polyhydramnios is pronounced 'poll-ee-hy-dram-nee-oss'. Hydrocephaly is pronounced 'hy-droh-keff-all-ee')
• Hydrocephaly ('water on the brain') i.e. an abnormally large fetal head due to excessive accumulation of fluid around the brain
• Placenta praevia
• Breech delivery in the previous pregnancy
• Abnormal formation of the uterus.