Breech presentation in pregnancy

Breech presentation at term (fetus is bottom first or feet first) occurs in less than 5% of pregnancies.  There are numerous causes of breech presention such as an abnormally shaped uterus (ie heart shaped), low amniotic fluid, high amniotic fluid, abnormal placenta placement, uterine fibroids, uterine scarring, fetal malformations, and decreased fetal mobility.  Random chance plays a signficant factor as well.
Most fetuses settle into their final position in the uterus around 34 weeks although certainly fetuses may switch positions even at 40 weeks or beyond.  If one is diagnosed with a breech fetus after 34 weeks,  management options include trying to turn the fetus to cephalic or head down position, scheduling a planned cesarean section, or attempting a vaginal breech delivery. 
External cephalic version is a procedure that an obstetrician will perform in the labor suite to attempt a change in the fetus's position.  A fetus is monitored via fetal heart rate monitoring usually for one half hour.  An ultrasound is performed to verify that adequate amniotic fluid is present.  An obstetrician will then push on the fetus through the maternal abdominal wall to attempt the fetus to "somersault" into position.  The success rate is variable and depends again on the factors that caused the breech position in the first place.  Risks associated with the procedure are infrequent (less than 5% of the time) but including fetal heart rate deceleration, placental abruption, rupture of amniotic membranes and stillbirth.  A fetus is monitored via fetal heart rate monitoring after the procedure as well.
Recent clinical trials have shown that fetuses born via cesarean delivery due to breech presentation instead of vaginal delivery tend to have decreased morbidity (illness or injury) or mortality (death).  This is because when a fetus is born via vaginal delivery in the breech presentation, the umbilical cord is more likely to get compressed, thereby decreasing the blood flow to the fetus.  The fetal head is also more likely to become stuck in the birth canal.  Most fetuses in breech presentation that do not convert to cephalic presentation after an attempted external cephalic version are born via cesarean section.
Some women choose to have a vaginal breech delivery which may be acceptable as long as they understand the above risks and are at a hospital where obstetricians are skilled in breech deliveries. For more information, please look at the following links:
http://www.ncbi.nlm.nih.gov/pubmed?term=11052579
http://www.acog.org/~/media/F65E95C3644349F1AEFABBB8B560CBE5.ashx