This will probably explain it better than me....
Labour is nearly always shorter and easier if your baby is in a head-down position, with the back of his head slightly towards the front of your tummy. This is called an anterior position. Most babies get into this position by the end of a pregnancy.
In an anterior position, your baby fits snugly into the curve of your pelvis. During labour, your baby will curl his back over and tuck his chin into his chest. Your labour and birth should progress easily if your baby is in this position because:
The top of your baby's head puts rounded, even pressure on the neck of your uterus (cervix) during contractions. This helps your cervix to widen and helps to produce the hormones you need for labour.
At the pushing stage, your baby moves through your pelvis at an angle so that the smallest area of his head comes first. Try putting on a tight polo-neck top without tucking in your chin and you'll understand how this works!
When your baby gets to the bottom of your pelvis, he turns his head slightly so that the widest part of his head is in the widest part of your pelvis. The back of his head can then slip underneath your pubic bone. As he is born, his face sweeps across the area between your vagina and back passage (the perineum).
What's a posterior position?
A posterior position is where your baby has his head down, but the back of his head is against your spine. By the time labour starts, about one baby in 10 is in this back-to-back or posterior position.
Most back-to-back babies are born vaginally, but this position can make labour more difficult for you, particularly if your baby's chin is pushed up rather than tucked in. As a result:
you may have backache as your baby's skull is pushing against your spine
your waters may break in early labour
your labour may be long and slow
you may feel like pushing before your cervix is fully dilated
When your baby gets to the bottom of your pelvis, he'll need to turn through almost 180 degrees to get into the best position to be born.
This can take quite a while, or your baby may decide he's not going to turn at all! In this case, he will be born with his face looking up at you as he emerges. He might need forceps or ventouse to help him out.