As term approaches, your baby will start to move into position ready for birth. From around 30 weeks, your GP or midwife will usually assess this by palpating your abdomen. This information, which is written in your antenatal notes, can indicate the approximate timing of the birth and, possibly, the method of delivery.
There are three elements to take into consideration when your baby's position is assessed:
- Lie: whether the baby is longitudinal (vertical), transverse (across) or oblique (diagonal) in your uterus
- Presentation: which part of the baby lies closest to the cervix and therefore the birth canal (as this will be the part that 'presents' itself first during the birth)
- Engagement: when the head or bottom starts to descend, or 'engage', inside the pelvic cavity. LieAt term, 99 per cent of babies lie longitudinally in the uterus, with either their head or their bottom near to the cervix. The remaining one per cent are lying either horizontally (transverse) or diagonally (oblique). All non-longitudinal labours should take place in hospital and, unless the baby turns at the last minute, are highly likely to be delivered by Caesarean section.PresentationThere are two presentations in a longitudinal lie: cephalic (where the baby is head down), which accounts for 95 per cent of presentations at term, and breech (where the baby is bottom down), which accounts for 4 per cent; in a non- longitudinal lie there will be no presenting part. However, the exact position in a cephalic presentation is an important factor, because certain ones may make labour slower.In pregnancy, the term 'position' refers to how the baby's spine and the occiput are placed relative to the inner wall of the uterus - 'occiput' is the anatomical term for the posterior (back) portion of the head or skull. There are several different positions and your baby may take up all of these during the last week of pregnancy and in early labour:
- Anterior (in front): The baby's spine is lying against your abdomen, and the baby is, in effect, looking backwards
- Posterior (to the back): The baby's spine is lying against yours, and your baby is facing the front of your abdomen, looking forwards
- Lateral (to the side): The baby's spine is to one side or the other, the most common position. In anterior (occiput-anterior, or OA) and posterior positions (occiput-posterior, or OP), the baby is usually facing a little to the right or to the left; it is rare for them to be facing directly towards the back (direct anterior) or front (direct posterior). All lateral descriptions refer to the mother's left or right. The anterior position is ideal for a cephalic labour, and this together with a lateral position, is considered normal.When a baby is in a posterior position, with its back towards yours, a first labour may be slower: this is because the presenting part is the widest part of the head, not the occiput (which would be preferable), and the cervix tends to dilate more slowly as the fetal head does not fit so closely. This position is also associated with a slower descent through the birth canal.Most babies do, nevertheless, rotate and are born face backwards, although this often takes longer in a first labour. However, it is still possible for the baby to be born face forwards.Around 30 per cent of premature babies (those born before 37 weeks) are breech, but even if a baby is breech at 36 weeks, they can still turn to a cephalic presentation, even though space is tighter during the last few weeks of pregnancy.Second and subsequent babies are more likely to be breech, because there is usually more room inside the uterus (as the muscles have stretched once already), which enables them to keep changing position right through to the end of the pregnancy. Breech presentation is also more common in women with fibroids and in those carrying very large or very small babies.By the last month of pregnancy, you will be having antenatal visits every two weeks, and at each one, your baby's head will be palpated to assess how far it is engaged. Usually, an external abdominal examination is enough to note this progress but, occasionally, an internal examination or an ultrasound scan will be required if there is any doubt about whether the head or the bottom are presenting.Even if it is confirmed that your baby's head is engaged, this does not mean that you are about to go into labour, especially if this is your first baby and you are still in the 36th week. It simply means that your baby is moving into position ready for birth.your baby's head.EngagementEngagement is the term used to describe the baby's descent into the pelvic cavity. In first pregnancies, this tends to start at around 36 weeks, whereas in subsequent pregnancies, engagement is usually later and may not even occur until the onset of labour itself. Early engagement in a first pregnancy happens because the uterine muscles are tighter and exert greater downward pressure on the baby. There are several levels of engagement, and these will be noted in your medical notes.This is an edited extract from One Born Every Minute: Expecting a Baby? by Dr Penelope Law (Quadrille, £25).Text © 2013 Dr Penelope Law