At one point in the ante-natal class, our guide explained the concept of posterior presentation at birth. While everyone is familiar with breech birth, where the legs come out first instead of the head and has obvious dangers, posterior delivery can also introduce complications. Essentially, the baby can be lying on its back instead of on its side, and this orientation can prevent the baby from being able to emerge - and lead to a long and painful labour while waiting for the baby to turn 90 degrees.
The room was then advised that if a mother-to-be wants to avoid this scenario, she should try to avoid lying on her back too much and sit up straight rather than slouching.
Every woman suddenly sat upright.