Occiput Posterior refers to the position of the fetal head during the latter stage of pregnancy. Occiput Posterior is a common cause of posterior labour, also known as back labour.
The occiput is the skull bone at the back of the head. The position of the fetal head during pregnancy is defined relative to the occipital bone. At the end of pregnancy, the head is down (cephalic presentation). The fetal back lines up on the maternal left or the maternal right. Usually fetal kicking is felt on the other side to where the fetal back is.
If baby is looking across from one side of the pelvis to the other, then the occiput is on the side where the back is and this position is called occiput transverse. If the fetal head is leaning backward with the fetus looking up, then the back of the head is down towards the pelvis and the maternal back. This is called Occiput Posterior. It is not the back of the baby that is posterior. The fetal back is on one maternal side or the other. It is the back of baby’s head that is posterior.
At any one time, almost a third of babies will be in the Occiput Posterior position so it is considered a variant of normal. When the fetal head is in the posterior position the head is deflexed on the neck so it takes up more space than if the head is flexed with the fetal chin close to the fetal neck.
Prior to labour this can be associated with more pelvic pain and lower back pain. During labour this position can be associated with a slower labour and significant lower back pain.
During labour the head will descend through the pelvis in either the Occiput Transverse position or the Occiput Posterior position. At the end of labour, when the fetal head comes through the bottom of the pelvis to come out the vagina, the head will turn so that is facing downward, or Occiput Anterior. After the head comes out of the vagina it will turn back to transverse or posterior and the shoulders will come out on one side or the other.
Most babies that are in the Occiput Posterior position prior to labour will turn during labour and come out of the vagina in the Occiput Anterior position. Some babies will start labour in the Occiput Transverse position, turn to Posterior during labour and then turn back to Occiput Anterior. Some women who have successful vaginal deliveries will have a baby in the Occiput Posterior position for part of all of their labours.
The position of the fetal head in the pelvis is determined by the relative size and shape of the fetal head relative to the size and shape of the maternal pelvis. Maternal position changes such as leaning forward have not been shown to change the fetal head position within the pelvis. The fetal head may be in a deflexed position due to a fetal hand or loop of umbilical cord under the fetal chin and this may predispose to an Occiput Posterior position.
With the Occiput Posterior position, there is a slightly increased chance of spontaneous rupture of membranes prior to labour and of labour not progressing so that a Caesarean Section is required for delivery. Sometimes if the fetal head is too large to fit through the pelvis, the position of the head will become Occiput Posterior in an attempt to descend deeper into the pelvis. If the fetal head is in the transverse or posterior position at the end of labour and not making the turn to anterior, then this makes pushing more difficult. A vacuum extraction may be needed to flex the head so that pushing will result in the head turning to anterior and being