Is my baby in the right position?

This is a very common question to be asked by a patient in advance pregnancy.  Occipito-Posterior Position

Usually when a patient asks me this question she is asking if her baby is in an occipito-anterior position.

Occipito-anterior is when the back of baby’s head (the occiput) is toward the pregnant woman’s front (or anterior).

This contrast to this occipito-posterior position is when the back of baby’s head is towards the pregnant woman’s back and the baby’s front is towards her front.

A pregnant woman asks the question ‘Is my baby in the right position?’ usually because she has heard that an occipito-posterior position of her baby in labour and delivery is associated with a greater likelihood of a more difficult labour and delivery.

Throughout the pregnancy, which way a baby is turned is not relevant as baby can move. This is particularly so if it is not the first ongoing pregnancy as the uterus is not as tight around the baby.

Most babies are in an occipito-transverse position when the baby’s head enters the pelvis. That means the baby’s is looking across the abdomen with the back of the baby’s head (the occiput region) neither anterior nor posterior. Some will be in an oblique occipito-anterior position and some in an oblique occipito-posterior position. It is not likely that a normal sided baby will be in a direct occipito-anterior position or direct occipito-posterior, as direct position are a consequence of internal rotation with descent through the birth canal during labour and delivery.

At the onset of labour about 10-20% of head down babies are in an oblique occipito-posterior position. Right occipito-posterior is more common than left occipito-posterior.

Of these 10-20% of babies in oblique occipito-posterior position with internal rotation during labour and delivery most turn to an anterior position.

About 65% of this 10-20% of all head down babies who start labour in an oblique occipito-posterior position will deliver in a direct occipito-anterior position. That means most babies in an occipito-posterior position at the start of labour will deliver in the occipito-anterior position.

About 20% of the 12% start to rotate to occipito-anterior but get stuck at occipito-transverse. We call this ‘deep transverse arrest’.

The rest about 15% of the 10-20% rotate to direct occipito-posterior and deliver in the occipito-posterior position.

The other day I had to do a vacuum delivery for persistent occipito-posterior where the mother could not push baby out. The baby’s head was delivering as a posterior and spontaneously turned to occipito-anterior as it started to crown with the suction cap in place. The husband was amazed.

Because of the irregular shape of the pelvic canal and the relatively large dimension of the mature baby’s head not all diameters of the head can pass through all diameters of the pelvis. There needs to be adaptation of the shape and position of baby’s head so baby’s head can fit through the various segments of the pelvis.  The steps are engagement, descent, flexion, internal rotation, extension, external rotation, and expulsion. These steps happen are not independent but happen in association with each other.

If the baby’s head enters the pelvis in the oblique occipito-anterior position, there is 45 degrees of rotation of the head to achieve a direct occipito-anterior position for delivery

Rotation of Baby's Head through PelvisIf the baby’s head enters the pelvis in the oblique occipito-posterior position, there needs to be much more internal rotation of the head – 135 degrees, to achieve a direct occipito-anterior position for delivery. But as stated this spontaneous rotation with descent and flexion does happen in most cases when the baby’s head enters the pelvis in an oblique occipito-posterior position.

Various factors including poor contractions, an abnormal shape to the pelvis or uterus, placental location, excess resistance from pelvic floor muscles and a large baby’s head in relation to the size and shape of the pelvis can prevent this internal rotation from happening.

With an occipito-anterior position with descent the flexion the baby’s head. Flexion means baby’s head tilts forward so baby’s chin presses baby’s chest so the minimum head diameter is presenting as the baby descends through the birth canal.

With a persisting occipito-posterior position these same forces that caused flexion with an occipito- anterior position mean that with rotation posteriorly and not anteriorly there is not flexion.  With a persisting occipito-posterior position the baby’s head tends to be deflexed. Being more deflexed means there is a larger diameter of head is presenting as baby descends through the birth canal.

Possible challenges with an occipito-posterior position of baby.

  • Going overdue. Occipito-posterior position in late pregnancy is associated going overdue. Going overdue compounds the challenges as the more overdue you go the larger is your baby and so with greater size the more difficult is likely to be labour and delivery.
  • Rupture of membranes before labour. Occipito-posterior position is associated with spontaneous rupture of membranes before the onset of labour and often with the head still being high, not engaged and not applied to the cervix.
  • A longer and more difficult labour. The deflexed head means the head is not as well applied to the cervix. If baby’s head is not well applied to the cervix, there is slower cervical dilatation and slower descent, and less efficient contractions and al longer labour. As well these factors mean it is harder for internal anterior rotation to occur.
  • Foetal distress. With a longer and more difficult labour there is more risk of foetal distress.
  • Backache. A persisting occipito-posterior usually causes much more backache in labour than an occipito-anterior position.
  • Desire to push too early. A persisting occipito-posterior will usually result in a desire to bear down and push early and sometimes before you are fully dilated. That urge should have discouraged. You should not push until you are fully dilated and baby’s head is well down.
  • Increased need for Caesarean section With a deflexed head, there is much more likelihood of an obstructed labour and the need for Caesarean section because of lack of progress in first stage labour, lack of descent in second stage labour, and the likelihood of a more difficult operative delivery in second stage labour. These scenarios can occur even though baby is normal size, or even less than normal size, and does not have a large head and the pelvis is a normal size and shape. As well there may be need for a Caesarean section because of foetal distress.
  • Increased need for operative vaginal delivery. There is increased likelihood of needing an operative vaginal delivery because of lack of progress with pushing in second stage labour and/or foetal distress in second stage labour.
  • Perineal and vaginal tearing. As there is a greater head diameter presenting the likelihood of more perineal and vaginal tearing with delivery is greater than with an occipito-anterior position baby.

Management that can help

Hopefully if baby’s head is in an oblique occipito-posterior position at the onset of labour it will rotate spontaneously to direct occipito-anterior position. But measures can be taken to help with an occipito-posterior position.

  • Don’t go too far overdue so baby does not get too big. A larger size for baby will only compound the possible challenges ahead. But so often in this context the cervix is long and closed and baby’s head is high at 40 weeks. All that means a successful induction is much less likely. In this context, some women will opt for a Caesarean section delivery rather than a potentially very difficult labour and delivery.
  • An early epidural in labour should be considered. An early epidural will mean you don’t need to experience the increased pain, especially backache, usually associated with an occipito-posterior. It will also help you relax. Relaxing can facilitate progress in labour.
  • Syntocinon augmentation of labour may be needed to maximise the efficiency of the uterine contractions and so increase the likelihood of spontaneous internal rotation of baby’s head. If there is no rotation to a direct occipito-anterior position, then Syntocinon augmentation will facilitate the delivery in the occipito- posterior position. It will mean a safer more efficient delivery and less likelihood of needing a Caesarean section or operative vaginal delivery.
  • You may be encouraged to change positions in labour and for delivery to help facilitate internal rotation of the baby’s head to occipito-anterior.
  • If you have a desire to bear down and push early, then an injection of Pethidine can help overcome this.
  • There is a greater likelihood of needing an episiotomy to control and so minimize the trauma to the vagina and perineum with a direct occipito -posterior position.
  • If an operative vaginal delivery is needed it is much safer for you and your baby to have this done by an experienced obstetrician. When the baby’s head is in a transverse, or posterior position then it is a potentially much more dangerous and difficult vaginal delivery that requires more skill to optimize your safety and your baby’s safety. If you need an operative vaginal delivery and you are not my patient, then ask your doctor about this.

Finally, persisting occipito-posterior position is more common in first labour. I believe this is because of the greater muscle tone of the pelvic floor muscles. If a woman persists in having babies in an occipito-posterior position, then it is possible she has an unusual shape to her bony pelvis which predisposes her to this.

Posted by Dr Gary Sykes on -

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