The position of the baby lying at rest in the uterus is known as the foetal position. The head is bent forwards with the chin resting on the chest, the arms are crossed so that the right hand rests on the left shoulder and the left hand rests on the right shoulder. The legs are bent at both the hip and the knee, so that the feet are crossed over the genitalia. The feet themselves are turned inwards so that they do not protrude outside the line of the body. The baby does, of course, move within the uterus and, especially in midpregnancy, it has great freedom to stretch and extend both its arms and legs. Frequent movement of all its muscles is essential in order to promote and maintain their development.
The ‘lie’ of the baby refers to the manner in which it is lying in the uterus.
If one end of the baby is lying over the pelvic brim or is engaged within the pelvis itself so that its spine is almost parallel to the mother’s spine, then the lie is stated to be longitudinal. In early pregnancy the baby can lie in any position and is free to twist and turn around inside the uterus as it wishes. At about the 28th week it begins to assume the longitudinal lie with one end lying immediately over the pelvic brim. In approximately 98 per cent of all pregnancies the lie is longitudinal.
In approximately 1.5 per cent of instances the lie is oblique where the lower part of the baby lies just above the groin with its spine lying obliquely in relation to the mother’s spine. This usually occurs only in women who have had several children and in whom the muscles of the abdominal wall‘have been slackened by repeated pregnancies, or where there is an excessive amount of fluid and therefore the baby is allowed to move around inside the uterus with comparative freedom. In nearly every instance an oblique lie will be converted spontaneously to a longitudinal lie at or before the onset of labour.
In approximately 0-5 per cent of pregnancies the spinal column of the baby lies at right angles to that of the mother. This may happen when a woman has had many pregnancies which have resulted in slackness of both the abdominal wall and the uterus, or in the presence of an excessive quantity of amniotic fluid. In very rare instances transverse lie may be caused by fibroids, or any other space-occupying tumour or cyst which prevents the baby from assuming a longitudinal lie, and it is corrected to a longitudinai lie by the doctor performing external version either before or immediately after the onset of labour. Very occasionally this may be impossible and Caesarean section is performed. Transverse lie, however, is a rare complication.
Presentation of the Baby
The presentation of the baby refers to that part of the baby which overlies the brim of the pelvis or the entrance to the birth canal. Nearly all babies are eventually converted into a longitudinal lie so that either the head or the bottom of the baby presents at the pelvic brim and the presentation is either head first (cephalic) or bottom first (breech).
At about the 28th week of pregnancy most babies are breech presentations with the head lying in the fundus, or upper part, of the uterus near the ribs. At about the 32nd week in a woman having her first baby, or at about the 34th week in a woman having subsequent children, the baby turns round spontaneously so that its head presents and it becomes a cephalic presentation. Neither the reason for this somersault nor the mechanism used to effect it is known. Ninety-six per cent of babies eventually assume a cephalic presentation, while the other 4 per cent remain as breech presentations.
The position of the baby’s back is also important because a well flexed baby will always lie with its back to the front of the abdomen in what is known as an ‘anterior’ position. The back may be just to the right or to the left of the mid-line. A posterior position is when the baby’s back is towards the mother’s spine and it may, similarly, be right or left. A posterior position, which sometimes occurs in a first pregnancy, is not very favourable because the baby cannot assume an attitude of complete flexion, so that the head does not fit satisfactorily into the pelvic brim and may not engage, resulting in premature breaking of the waters and sometimes a long labour.
In cephalic presentation the head presents. It is normally well flexed, which means that the chin is pressing down towards the chest. In 99 per cent of cephalic presentations the posterior part of the head, or vertex, is the part which actually descends first and will be delivered first. Such presentations are frequently known as vertex presentations. In the remaining 1 per cent the head is extended, so that instead of the baby having its chin tucked on its chest its head is bent as far back as possible, so that it looks directly into the pelvis and the birth canal. This is known as a face presentation. From a purely mechanical point of view a face presentation is only slightly more difficult than a vertex presentation. Whatever part of the baby’s head presents a certain amount of swelling, known as the caput, forms over the area which lies immediately above the dilating cervix. This swelling can be fairly extensive and may distort the head, or the lips, nose and cheeks of a face presentation, so that the baby looks quite ugly immediately after delivery. The swelling soon disappears and the baby’s features rapidly return to normal with no ill effect.
As stated above, the majority of babies are breech presentations until approximately the 32nd week of pregnancy, after which they spontaneously rotate through 180° to cephalic presentations. Approximately 4 per cent of babies, however, fail to undergo this rotation and remain breech presentations.
External Cephalic Version
It is very much easier and safer for both the mother and her baby if the child is delivered head first. An important part of antenatal care is to ensure that the baby assumes the correct position at the right time and antenatal attendances are therefore usually arranged at 2-weekly intervals from the 30th week. If by the 32nd week in a primigravida, or by the 34th week in a woman having a subsequent pregnancy, the baby has not yet assumed its cephalic presentation, the obstetrician may decide to turn the baby from a breech to a cephalic presentation, and this is known as external cephalic version (E.C.V.). This sounds very dramatic but is in fact extremely simple and entirely free from pain or discomfort.
The woman lies on an examination couch with her feet slightly raised. She is asked to relax completely. Her abdomen is gently powdered and the doctor, using both hands, will slowly rotate the baby by exerting gentle pressure on it so that the baby’s body is pushed up on one side of the uterus and the baby’s head is gently pulled down the other side. This is frequently practised in antenatal clinics and has helped reduce the incidence of breech presentation from its normal 4 per cent to less than 1 per cent. External cephalic version may be performed on several occasions if necessary. There is no danger of the baby becoming entwined within his umbilical cord or of knots forming during this procedure. Remember that earlier in pregnancy the baby is free to twist, turn and rotate within the uterus without becoming entangled in its cord. After this manoeuvre the woman remains on the couch for a few minutes and the position of the baby is confirmed by the doctor. It is usual to see the woman one week later to ensure that the baby has remained in its new presentation.
When an excessive amount of water is present the baby may not assume its final presentation within the uterus until very late in pregnancy and in these circumstances external cephalic version may be performed much later than the 32nd or 34th week.
External cephalic version can occasionally be very difficult to perform, partly because the uterus will not relax and also partly because of the inability of the woman to relax her abdominal wall. There might also be a reduced amount of amniotic fluid present. If the uterus or abdominal wall will not relax sufficiently the woman will be asked to return a week later when she might be more relaxed. If external cephalic version is still not possible the use of anaesthesia may be considered. The woman is admitted to hospital overnight and the baby gently turned round under the influence of the complete relaxation of a general anaesthetic.
Occasionally, however, it is impossible to rotate the baby so that breech presentation persists and breech delivery has to be undertaken. The buttocks present over the cervix and dilatation of the cervix occurs as in cephalic presentation. A swelling, or caput, will form mostly in the region of the genitalia, so that these may be considerably distorted at birth. As with cephalic presentation, this swelling rapidly disappears and the genitalia return to normal within 48 hours.