Any deviation from the normal position of the fetus for delivery
In most normal pregnancies, the fetus settles into the mother’s pelvic cavity from week 36 onwards, ready for labour and birth. About 8 in 10 fetuses settle head downwards, facing the mother’s back, with the chin resting on the chest. In this presentation, the fetus is in the optimum position for birth, and a normal vaginal delivery is usually possible. All other fetal positions are considered to be abnormal presentations and may cause problems during labour.
When a fetus is lying in an abnormal position in the uterus, a vaginal delivery may be possible, but the labour may be prolonged (see Problems during delivery). If the fetus becomes stuck, it may need an assisted delivery or caesarean section.
Most abnormal presentations can be diagnosed before labour begins, and arrangements to deliver the fetus safely can be discussed in advance.
Abnormal presentation may occur if the fetus is able to move more freely within the uterus than usual, either because the fetus is small and does not fit closely into the pelvis (see Premature labour) or because there is excess amniotic fluid surrounding the fetus (see Polyhydramnios). There is also an increased risk of an abnormal presentation when there is more than one fetus in the uterus (see Multiple pregnancy and its problems).
Occasionally, the fetus cannot settle into the pelvic cavity because of an obstruction, such as the placenta lying low in the uterus (see Placenta praevia) or a noncancerous growth in the uterine wall (see Fibroids). An unusually shaped uterus may also contribute to an abnormal presentation.
The most common abnormal presentations are the breech position and the occipitoposterior position. Rarer types include face, compound, brow, and shoulder presentations, named according to the parts of the fetus that are over the dilated cervix at delivery.
In a breech birth, the fetus presents buttocks first. Many fetuses lie in a breech position before week 32 of pregnancy but most turn by 36 weeks. The 3 in 100 that do not turn are in one of three types of breech presentation. A complete breech is one in which the fetus is curled up. In a frank breech, the legs are extended and the feet are close to the face. In a footling breech, one or both feet are positioned over the cervix. Often one twin fetus is a breech.
At the beginning of labour, about 1 in 5 fetuses lies in an occipitoposterior position, which is head-down but facing the mother’s abdomen instead of her back. Most fetuses will turn at some stage during labour, but 2 in 100 do not and are still in this presentation when they are delivered.
In a face presentation, the neck of the fetus is bent backwards so that the face is positioned over the cervix. This type of presentation occurs in 1 in every 400 births. In a compound presentation, which occurs in 1 in every 700 births, an arm or leg lies over the cervix in addition to the head or buttocks. In a brow presentation, the fetus’s head is bent slightly backwards with the brow over the cervix; this occurs in about 1 in every 1,000 births. A shoulder or oblique presentation, in which the fetus lies across the uterus with its shoulder over the cervix, occurs in 1 in every 2,500 births.
If the fetus lies in an abnormal position just before delivery, there may be complications that place both the fetus and mother at risk. A fetus in the normal head-down position blocks the cervix and prevents the umbilical cord from passing out of the uterus before the fetus. Some abnormal presentations leave space for the cord to drop through the cervix when the membranes surrounding the fetus rupture. When this occurs, the cord may be compressed by the fetus, or, rarely, its blood vessels may go into spasm because of the drop in temperature outside the uterus. As a result, the fetus may be deprived of oxygen (see Fetal distress). This may cause brain damage or fetal death.
A breech delivery may cause problems if the legs and body of the fetus are able to pass through the cervix when it is not completely dilated, but the head becomes stuck. If, in a footling breech, one foot drops through the cervix, this may prompt the mother to try to push too early. An abnormal presentation may also increase the risk of the cervix or vagina being torn during delivery.
Normally, the position of the fetus in the uterus is assessed at each antenatal visit. Your doctor will usually be able to tell if your baby is lying in an abnormal position at the end of your pregnancy. If your doctor suspects an abnormal presentation, you will be given ultrasound scanning to confirm the position (see Ultrasound scanning in pregnancy).
Abnormal presentation is usually diagnosed towards the end of a pregnancy, and, if necessary, a caesarean section is arranged. A vaginal birth may be possible for some breech or occipitoposterior presentations. Sometimes, a mother with a fetus in a breech presentation is offered a procedure to turn the fetus around after week 37 of pregnancy. The doctor attempts to manipulate the fetus into the correct position by pushing gently on the wall of the abdomen. This technique is performed using ultrasound guidance. It does not require an anaesthetic and is successful in about 50 per cent of cases.
If the abnormal presentation was due to a structural problem, such as an unusually shaped uterus, there is an increased risk of recurrence in a subsequent pregnancy. However, if the presentation was caused by a condition associated with that particular pregnancy, such as placenta praevia, the risk of recurrence is not increased.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.