What causes babies to end up in the breech position?

Usually no reason: Twin pregnancies can be breech because of limited room in the uterus. Some babies may have a larger head than usual that does not fit easily in the mother's pelvis. Sometimes certain abnormalities of mother's pelvis or uterus may force baby to assume a breech position. Most cases there is no obvious reason.

Many factors: Causes for breech presentation include preterm babies, twins, polyhydramnios (excessive amniotic fluid), fibroids, uterine anomalies, or placenta previa. Many breech presentations have none of these risk factors.