Whilst it is fairly common for a preterm baby to change positions in the uterus frequently, by the third trimester most babies prefer to move into a “head down” position. This is thought to be because the top part of the uterus is roomier and allows more space for the baby to move it’s legs (and therefore keep it’s mother up all night!)
Turning “head down” for vaginal delivery is advantageous for a couple of reasons. Firstly, coming out head first allows the cervix to open completely to let the largest part of the baby out first so that the rest generally follows easily. Also, the head fits neatly against the cervix during labour, similar to an egg in an egg cup, which prevent the umbilical cord from slipping out first and being compressed.