It takes a fetus about 37 weeks to 42 weeks to develop in the mother’s womb normally and ready to be delivered. As the fetus grows in the womb, all the parts of the body becomes formed and can be seen on ultrasound scan or felt on palpation. The fetus normally grows “upside down” in the womb at labour, with the head facing the cervix, which is called vertex presentation (the head is the presenting part). As labour approaches, the fetal head descends in a flexed vertex position further down the the uterus and when the cervix is fully dilated, normal delivery takes place with the baby coming out head-first.
If for any reason the fetus reaches labour with the head not coming as the presenting part, the phenomenon is called fetal malpresentation. In the situation where the head is the presenting part but not coming in the normal flexed vertex position, it is called a malposition. A malpresentation or malposition of the fetus is considered as an abnormal labour which predisposes to several risks and is treated with care to ensure a successful delivery with both mother and child healthy.
Dangers of malpresentation or malposition
- Malpresented and malpositioned parts due to the force of uterine contractions make the forewaters (chorionic fluid) which is not protected be forced through an incompletely dilated cervix.
- Membranes rupture early and umbilical cord may prolapse past the presenting part.
- Contractions can become irregular and poorly sustained after rupture of membranes and dilatation of the cervix is likely to cease temporarily.
- Some malpresentations can obstruct labour and uterine rupture can occur if not recognised early.
TYPES OF FETAL MALPRESENTATION/MALPOSITION
This is a malposition of the head which occurs in about 13% of all vertex presentation. The presenting part is the head (vertex) but the denominator (also referred to as leading part) is the occiput. The causes of this malposition include pendulous abdomen, anthropoid pelvic brim, android pelvic brim, a flat sacrum, and a placenta situated on the anterior uterine wall.
The diagnosis of occipito-posterior malposition is best through ultrasound but can also be picked up clinically through the following findings:
- On palpation, the fetal head is postero-lateral and found free above the pelvic brim. The fetal back is on one side and may be difficult to identify but the limbs are felt at the front and give a hallowing above the head.
- The fetal heart is heard best well out in the flank but descends to just the pubis as the head rotates and descends.
- Vaginal examination may show early ruptured membrane before labour is established. If the membranes are intact, they may protrude through the cervix, giving a finger-like forewaters or may fill up the vagina obscuring the presenting part.
In the management of occipito-posterior malposition, analgesia can be given during painful labour and catheter should be inserted if there is retention of urine. The baby can be delivered by spontaneous vaginal delivery which may require episiotomy.
This occurs in 1 in 300 labours. The presenting part is the face and the denominator is the mentum or chin which may be anterior (mentoanterior) or posterior (mentoposterior). The most common causes are high parity, fetal abnormalities and thyroid enlargement.
On palpation, the cephalic prominence is very easily felt in mentoposterior face malposition on the same side as the back. It may be difficult to locate and hear the fetal heart sounds but in a mentoanterior position, the fetal heart is easily heard on the chest. Vaginal examination may show early rupture of membranes. The supraorbital ridges, nasal bridge and alveolar margins within the mouth can be recognised.
Management during labour involves analgesia and bed rest during the first stage. Vaginal delivery is possible in a mentoanterior position but a cesarean section is required for a mentoposterior positioned fetus.
This is similar to face malposition bit in this case, the face is not fully extended and so the eye brow and forehead becomes the leading part or denominator. It has the same causes as face malposition.
On palpation, the can be felt above the pelvic brim and the cephalic prominence is on the same side as the back. Vaginal examination may show early rupture of membranes and cord prolapse. The forehead,orbital ridge and nasal bridge with the fontanelle behind can be encountered by the examining finger. Brow presentation is suspected if there is non engagement of the fetal head after the rupture of membranes in a woman who has had previous easy delivery. Ultrasound is very helpful to the diagnosis.
Management of brow malposition is by cesarean section. However, vaginal delivery is possible if the baby is very small and head has engaged and there is no evidence of cephalopelvic disproportion or other contraindications.
It occurs in about 2.5% of labour cases and about 25% of all pregnancies have breech malpresentation before the 30th weeks of gestation which makes preterm delivery a major concern. In breech malpresentation, the fetus descends with the buttocks and the limbs fully flexed (full breech malpresentation) or the limbs fully extended (Frank breech malpresentation) especially in a primigravida. The causes include, fetal malformations, hydramnios, lay uterus and pendulous abdomen, abnormal shape of pelvic brim or uterus and placenta praevia.
To diagnose this, palpation will reveal a longitudinal lie, firm lower pole, limbs on one side and hard head at the fundus on top near the ribs of the mother. Fetal heart sounds are heard best above the umbilicus. Vaginal examination will show no head in the pelvis. The soft buttocks and hard sacrum or feet may be in the pelvis and the leading parts. Ultrasound scan is used for confirmation.
To manage this malpresentation, an external cephalic version, where the obstetrician attempts to turn the fetus around carefully from the abdomen, can be done at 34 weeks or more unless contraindicated. Analgesia may be required during labour and if external cephalic version does not work, vaginal delivery can be done in some cases where the woman presents in advanced labour, particularly if the legs are fully extended and no other contraindications. Otherwise cases of breech malpresentation require cesarean section.
Transverse or oblique malpresentation
The fetus may lie with its long axis transverse or oblique in the uterus where the point of the shoulder is usually the leading part. It occurs in 1 in 500 labours and can easily be corrected. The causes include lax uterus, preterm labour, polyhydramnios, or multiple pregnancy, contracted pelvis, placenta praevia and fibroid. Multi parity is by far the most common cause of transverse or oblique lie.
In the diagnosis of transverse lie, the uterus appears asymmetrical and broader than usual with the fundus lower than expected for the duration of pregnancy. The hard round fetal head is felt in one iliac fossa with the softer breech on the opposite side. Vaginal examination may show early rupture of membranes and the presenting part is too high to be felt. When the cervix is dilated, an arm or umbilical cord may prolapse into the vagina.
The management of this malpresentation is by external cephalic version in early labour if membranes are still intact. Other than that, a cesarean section is the safest route of delivery.
K. P. Hanretty : “Fetal malpresentation ” Obstetrics illustrated (6th edition)
Geoffrey Chamberlain : “Fetal malpresentation ” Obstetrics by Ten Teachers (16th edition)