A breech birth (also known as breech presentation) refers to the position of the baby in the uterus such that it will be delivered buttocks first as opposed to the normal head first position. Certain factors can encourage a breech presentation. These include multiple (or multifetal) pregnancy (twins, triplets or more), excessive amounts of amniotic fluid, hydrocephaly, anencephaly, very short umbilical cord, and some uterine abnormalities. Babies with congenital abnormalities are more likely to present by the breech. It is postulated that the baby normally assumes a head down presentation because of the weight of the baby's head. As the mass of the fetal head is the same as that of the pelvis, it is more likely that the enlarging fetus is more and more restricted in its movements, and simply becomes entrapped. The shape of the uterus is a more likely determinant of the final fetal presentation as uterine shape anomalies are strong predictors of breech presentation and other malpresentations.