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.
When recognized by the spliceosome, the sequence between the original and duplicated AGGT will be spliced, resulting in the creation of an intron without alteration of the coding sequence of the gene. Double-stranded break repair via non-homologous end joining was recently identified as a source of intron gain when researchers identified short direct repeats flanking 43% of gained introns in Daphnia. These numbers must be compared to the number of conserved introns flanked by repeats in other organisms, though, for statistical relevance. For group II intron insertion, the retrohoming of a group II intron into a nuclear gene was proposed to cause recent spliceosomal intron gain. Intron transfer has been hypothesized to result in intron gain when a paralog or pseudogene gains an intron and then transfers this intron via recombination to an intron-absent location in its sister paralog. Intronization is the process by which mutations create novel introns from formerly exonic sequence. Thus, unlike other proposed mechanisms of intron gain, this mechanism does not require the insertion or generation of DNA to create a novel intron.The only hypothesized mechanism of recent intron gain lacking any direct evidence is that of group II intron insertion, which when demonstrated in vivo, abolishes gene expression. Group II introns are therefore likely the presumed ancestors of spliceosomal introns, acting as site-specific retroelements, and are no longer responsible for intron gain.