Then, the shoulders follow the same path as the hips did. At this time the baby usually turns to face the mothers back. Next occurs external rotation, which is when the shoulders emerge as the babys head enters the maternal pelvis. The combination of maternal muscle tone and uterine contractions cause the babys head to flex, chin to chest. Then the  back of the babys head emerges and finally the face. Due to the increased pressure during labour and birth, it is normal for the babys leading hip to be bruised and genitalia to be swollen. Babies who assumed the frank breech position in utero may continue to hold their legs in this position for some days after birth. Cesarean or vaginal delivery
When a baby is born bottom first there is more risk that the birth will not be straight forward and that the baby could be harmed. For example, when the babys head passes through the mothers pelvis the umbilical cord can be compressed which prevents delivery of oxygenated blood to the baby. Due to this and other risks, babies in breech position are often born by a planned caesarean section in developed countries.Caesarean section reduces the risk of harm or death for the baby but does increase risk of harm to the mother compared with a vaginal delivery. It is best if the baby is in a head down position so that they can be born vaginally with less risk of harm to both mother and baby.
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 obstetricians 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 babys 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. 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 (for instance, cerebral palsy) or death. It has been suggested that a fast vaginal delivery would mean the risk of stopping babys oxygen supply is reduced. However, there is not enough research to show this and a quick delivery might cause more harm to the baby than a conservative approach to the birth.Injury to the brain and skull may occur due to the rapid passage of the babys head through the mothers pelvis. This causes rapid decompression of the babys 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.