Every Which Way But...
Occasionally I'll encounter a baby that's in a crazy position, such as transverse (whole body sideways) or shoulder-first. C-section is the safest way to address this malpresentation. It is just common sense: the forces of labor will crunch a baby that's not pointing straight down.
And Then There's Twins!
If you think the placenta can crowd out an adequate space, imagine what an extra baby does! Twins will compete for the most comfortable space, but usually there's a membrane that will separate them and favor one to be lower. The big risk here, though, is a breech baby that's first, compared to the head-first second twin. In this positioning, the head of Baby One (the breech) may sit just above the head of Baby Two (the higher of the babies).
When labor ensues for a vaginal delivery, it's possible to have the horror of interlocking heads. Needless to say, a breech/head-first presentation of twins necessitates a C-section. But head-first/head-first twins can deliver vaginally, as can head-first/breech. (Except, once again, when an OB just doesn't feel comfortable with this type of delivery; as is the case, once again, with me.)
There can be a considerable wait sometimes for Baby Two to descend, and with this there's the possibility of a prolapsed cord, which could cut off oxygen to this twin. Because of this, although not always, doctors will opt to deliver twins via C-sections.
The Happy, Healthy Delivery
Delivery of a baby (or babies) requires knowledge of the baby's position. With third- and fourth-generation antibiotics and advanced surgical technique, the risk to the mother from a C-section is now outweighed by the benefit to the baby when there's positioning that would make vaginal delivery hazardous.
In contrast to the dangerous times of just a generation ago, we no longer need to seek heroic vaginal deliveries for these babies. And even using this way out, the C-section rate can still be kept low by waiting for proper descent of the baby's head into the maternal pelvis before administration of an epidural.