Active Birthing
For which of you desiring to get something out of a bottle decides to lay the bottle on its side and squeeze it? Do you not turn it upside down such that it is erect with the tip pointing downwards? In that manner, your endeavour is aided by gravity and thus, less arduous. Why then do you choose to birth lying flat on your back?
Aside from working against gravity, you would also be working against your body. As your baby descends through the birth canal, your pelvis and lower back open out to make room. The tailbone (coccyx) moves to create more space. Lying on your back impedes this process and reduces the dimensions of the birth canal. It can also increase your risk of bruising or even fracturing your coccyx. Plus, if the baby is in any other position than occipito-anterior, you need all the space you can get.
Stay upright and active as much as possible in labour and delivery. If you have to stay in bed, eg for monitoring, you can still kneel or stay on hands and knees. Side lying is another great option that leaves the coccyx free. If you must medically lie on your back (extremely rare), use cushions or towels under your lower back to keep your coccyx slightly off the bed and give some space.