I don’t have personal experience, but I can give you the party line.
For all who don’t know, breech babies are inherently high-risk for two main reasons. The first is the risk of umbilical cord compression during vaginal delivery, and the second is the risk of head entrapment.
In the vast majority of babies, the head is the largest (widest) part. The skull bones aren’t fused so the head can be squooshed up a bit during delivery, but the head is usually wider than the shoulders and the fetal chest. Occasionally it isn’t, which is when shoulder dystocia ensues. In a vaginal childbirth, your time and effort is spent delivering the head. Once the baby’s head is out, the rest of the baby’s body usually follows in 1-2 pushes. Therefore the umbilical cord (which obviously is at the level of the baby’s bellybutton) is compressed for a small fraction of a second as the body slithers out, and the baby is never oxygen-deprived. If the cord is compressed, it’s like a scuba diver whose air hose is pinched: no way to get oxygen, and distress very quickly ensues.
In a breech birth different parts of the baby present first. The best outcome is with a “frank breech”: the baby’s butt is presenting, with its legs flexed all the way up like a diver next to its head. Since the butt/hips is wide in diameter, and since the umbilical cord is protected by the fetal legs somewhat, it is safer to assume that once the body is delivered, there will be enough room for the head. Safer, but not totally safe. If the body is difficult to deliver, the cord can still be seriously compressed, and the head can become entrapped.
The worst position is a “footling breech,” where the feet come out first. The baby’s feet, legs and hips are narrow in diameter and are delivered quickly. The umbilical cord will be stretched up from out of the vagina, smooshed by the head which is still inside the uterus, and attached to the placenta inside. Serious cord compression and serious oxygen deprivation can result. Since the feet are so small, the baby’s body can be partially delivered before the mother is even completely dilated, which means there is little to no chance of delivering the head. [name]Even[/name] if she is completely dilated, she won’t yet have done the work of labor delivering the baby’s head-- that might be a couple of hours of pushing, all of which the baby is seriously oxygen-deprived. Lastly, as in any birth, tt may be the case that there is true cephalopelvic disproportion where the head simply won’t fit through the pelvis, and the head is truly entrapped. This is a life-threatening emergency and even a c-section cannot save the baby as the body is already outside-- cutting into the uterus would not help. [name]One[/name] of the only remedies is something called the Zavanelli Manuever, which is manually shoving the distressed baby back up into the uterus and then performing a c-section. This rarely works.
Vaginal breech birth had a mortality rate of nearly 3%. Footling breech birth, as a subset, has a mortality rate in some case series of 20-30%. This is a disconnect between mothers and doctors: mothers see the 3% and say “97% chance of it working? I’ll take those odds!” Whereas doctors are like “3% chance of death, where the overall intrapartum death rate for all babies (even preemies, congenital anomalies, etc) is 0.6/10,000? I don’t think so!” Many doctors refuse wholesale to perform vaginal breech births as they think it is an unacceptably high risk to the baby for no particularly good reason-- after all, that is a 50-fold increase in mortality.
ECVs are performed at full-term because there is a danger to them. [name]Both[/name] the danger of it simply not working, as well as the danger of placental abruption. They are only performed under continuous fetal monitoring and the mother must be prepared to accept an emergency c-section if complications arise. And as many posters noted, position at 35-36 wks is not necessarily final position; many babies do change position before engaging in the pelvis. Once they’re engaged, though, it’s usually final.