Traditional Breech Presentation
Breech refers to a feet- or buttocks-first presentation. This is a real thinking obstetrician's dilemma, because, as mentioned above, the largest part of the baby is the head. So delivery of the feet or buttocks first creates a scenario where larger and larger parts of the baby have to clear the pelvis. In other words, if the head won't fit out in a head-first baby, delivery can be effected via second choice–the C-section. But in a breech delivery, if the head won't fit out, the rest of the baby already has. The cord is out as well and is compressed in the birth canal on it's way up to the placenta. This is an emergency with such a bad outcome that most obstetricians feel that a breech presentation necessitates a C-section.
So why do some babies come out head first and others breech? Babies tend to seek the most comfortable position in their mothers' wombs. If the largest part of the baby is the head, then the baby will fidget and maneuver until the head gravitates to the largest space in the uterus. The most generous space is usually the lower uterus; and usually at 32 weeks this position will stick.
A breech usually occurs when there's a problem and the lower uterus is not the biggest space. For instance, a low-lying placenta can occupy enough space so that the there is more space higher in the uterus. Also, congenital abnormalities can make other parts of the baby the biggest body part, meaning that part will become the lowermost presenting part. In fact, in my training it was always a warning to check for abnormalities when there was a breech presentation.
Other Breech Presentations
In a frank breech, the buttocks are first. In a footling breech, one or both feet are first–the single-footling breech or the double-footling breech. The difference depends on whether the knees are bent or not. In both situations, the hips are flexed, but if the knees are straight, then the lower legs, along with the thighs, are bent over the baby's abdomen, resulting in the frank breech. If the knees are bent, then the feet are positioned back toward the cervix and the outside world.
Thankfully, most breeches are in that position for unknown reasons. Mostly normal breech babies are delivered by C-section, putting to rest the fear of congenital problems. Some brave obstetricians (I'm not one of them) deliver these babies, but this isn't acceptable unless the baby is estimated to be at least a pound less than the mother's previous largest baby, the baby is a frank breech, and the maternal pelvic measurements are generous.
Any breech that is a first baby should still be born by C-section, because it's easier to explain a possibly unnecessary C-section than to explain a baby that was traumatized by the too large after-coming head. Plus, many OBs believe that even successful vaginal deliveries of breech babies result in what are called soft neurological signs (not brain damage, but Attention Deficit Disorder, dyslexia, hyperactivity, and the like).