Certain factors can encourage a breech presentation. Prematurity is likely the chief cause. Twenty five percent of babies are in the breech position at 32 weeks gestation; this drops to three percent at term. The increasing baby size near term traps the fetus into the head down position normally. Pregnancies ending prematurely simply recruit more breeches before they can turn to head down. Factors predisposing to term breech presentation include:
It is postulated that the baby normally assumes a head down presentation because of the weight of the baby's head. As the mass of the fetal head is the same as that of the pelvis, it is more likely that the enlarging fetus is more and more restricted in its movements, and simply becomes entrapped. The shape of the uterus is a more likely determinant of the final fetal presentation as uterine shape anomalies are strong predictors of breech presentation and other malpresentations.
Researchers generally cite a breech presentation frequency at term of 3-4% at the onset of labour though some claim a frequency as high as 7%. When labour is premature, the incidence of breech presentation is higher. At 28 weeks' gestation 25% of babies are breech, and the percentage decreases approaching term (40 weeks' gestation).
There are four main categories of breech births:
As in labour with a baby in a normal head-down position, uterine contractions typically occur at regular intervals and gradually cause the cervix to become thinner and to open. In the more common breech presentations, the baby’s bottom (rather than feet or knees) is what is first to descend through the maternal pelvis and emerge from the vagina.
At the beginning of labour, the baby is generally in an oblique position, facing either the right or left side of the mother's back. As the baby's bottom is the same size in the term baby as the baby's head. Descent is thus as for the presenting fetal head and delay in descent is a cardinal sign of possible problems with the delivery of the head.
In order to begin the birth, internal rotation needs to occur. This happens when the mother's pelvic floor muscles cause the baby to turn so that it can be born with one hip directly in front of the other. At this point the baby is facing one of the mother's inner thighs. Then, the shoulders follow the same path as the hips did. At this time the baby usually turns to face the mother's back. Next occurs external rotation, which is when the shoulders emerge as the baby’s head enters the maternal pelvis. The combination of maternal muscle tone and uterine contractions cause the baby’s head to flex, chin to chest. Then the face emerges, and finally the back of the baby's head.
Babies who assumed the frank breech position in utero may continue to hold their legs in this position for some days after birth - this is normal.
Umbilical cord prolapse may occur, particularly in the complete, footling, or kneeling breech. This is caused by the lowermost parts of the baby not completely filling the space of the dilated cervix. When the waters break the amniotic sac, it is possible for the umbilical cord to drop down and become compressed. This complication severely diminishes oxygen flow to the baby and the baby must be delivered immediately (usually by Caesarean section) so that he or she can breathe. If there is a delay in delivery, the brain can be damaged. Among full-term, head down babies, cord prolapse is quite rare, occurring in 0.4 percent. Among frank breech babies the incidence is 0.5 percent, among complete breeches 4-6 percent, and among footling breeches 15-18 percent.
Head entrapment is caused the failure of the fetal head to negotiate the maternal pelvis. At full term, the bitrochanteric diameter (the distance between the outer points of the hips) is about the same as the biparietal diameter (the transverse diameter of the skull)- simply put the size of the hips are the same as the size of the head. The relatively larger buttocks dilate the cervix as effectively as the head does in the typical head-down presentation. The relative head size of a premature fetus is significantly greater that the fetal buttocks. If the baby is premature, it may be possible for the baby’s body to emerge while the cervix has not dilated enough for the head to emerge.
Because the umbilical cord—the baby’s oxygen supply—is significantly compressed while the head is in the pelvis during a breech birth, it is important that the delivery of the aftercoming fetal head not be delayed. The head only just fits through the pelvis, and if the arm is extended alongside the head, delivery will not occur. If this occurs, the Lovset manoeuvre may be employed, or the arm may be manually brought to a position in front of the chest. The Lovset Manoeuvre works by rotating the fetal body by holding the fetal pelvis. Twisting the body such that an arm trails behind the shoulder, it will tend to cross down over the face to a position where it can be reached by the obstetrician's finger, and brought to a position below the head. A similar rotation in the opposite direction is made to deliver the other arm. In order to present the smallest diameter (9.5 cm) to the pelvis, the baby’s head must be flexed (chin to chest). If the head is in a deflexed position, the risk of entrapment is increased. Uterine contractions and maternal muscle tone encourage the head to flex. If the birth attendant pulls on the baby’s body, this action may deflex the head.
Oxygen deprivation may occur from either cord prolapse or prolonged compression of the cord during birth, as in head entrapment. If oxygen deprivation is prolonged, it may cause permanent neurological damage or death.
Injury to the brain and skull may occur due to the rapid passage of the baby's head through the mother's pelvis. This causes rapid decompression of the baby's head. In contrast, a baby going through labor in the head-down position usually experiences gradual molding (temporary reshaping of the skull) over the course of a few hours. This sudden compression and decompression in breech birth may cause no problems at all, but it can injure the brain. This injury is more likely in preterm babies. The fetal head may be controlled by a special two handed grip call the Morisseau-Smellie-Veit manoeuvre or the elective application of forceps. This will be of value in controlling the rate of delivery of the head and reduce decompression.
Squeezing the baby’s abdomen can damage internal organs. Positioning the baby incorrectly while using forceps to deliver the aftercoming head can damage the spine or spinal cord. It is important for the birth attendant to be knowledgeable, skilled, and experienced with all variations of breech birth. In the United States, because Cesarean section is increasingly being used for breech babies, fewer and fewer birth attendants are developing these skills.
Injury may occur even if a birth attendant uses appropriate interventions during labour. A majority of full-term term frank breech babies would be born without problems even without assistance. However, in a minority of cases, expert assistance is needed for the baby to be born safely. This must be placed in perspective. It is this minority that determines the safety of the choice of vaginal delivery of the breech. A fetal death rate as low as 1% might be acceptable to some societies if a greater benefit could accrue. Take a country like the United States with a population of 300 million, and a 14.14/1000 birth rate, assume a 3% breech rate, and the aforementioned 1% mortality. This would result in an annual attributable death rate from breech delivery of 1,273 babies per year. Attributable death rate implies that the deaths occurred because of the selection of vaginal delivery and not from concurrent problems, such as congenital abnormalities or prematurity.
Various manoeuvres are suggested to assist spontaneous version of a breech presenting pregnancy. These include maternal positioning or other exercises. A study has shown that there is insufficient evidence as to the benefit of maternal positioning in reducing the incidence of breech presentation.
One large study has confirmed that elective cesarean section has lower risk to the fetus and a slightly increased risk to the mother, than planned vaginal delivery of the breech however elements of the methodology used have undergone some criticism.
The same birth injuries that can occur in vaginal breech birth may rarely occur in caesarean breech delivery. A Caesarean breech delivery is still a breech delivery. However the soft tissues of the uterus and abdominal wall are more forgiving of breech delivery than the hard bony ring of the pelvis. If a caesarean is scheduled in advance (rather than waiting for the onset of labor) there is a risk of accidentally delivering the baby too early, so that the baby might have complications of prematurity. With proper prenatal care, including first trimester ultrasound, this is theoretically impossible, and is indeed almost unheard of. The mother's subsequent pregnancies will be riskier than they would be after a vaginal birth (uterine rupture). The presence of a uterine scar will be a risk factor for any subsequent pregnancies.
Data on breech presentation reported by researchers at Oregon Health & Science University, Department of Obstetrics & Gynecology.
Oct 28, 2010; New investigation results, 'Are race and ethnicity risk factors for breech presentation,' are detailed in a study published in...