The institution decides what is best for you. Since a pregnant woman is a patient they wheel her away in a wheelchair separating her from anyone accompanying her. Then they give her an enema so that when the birth occurs poop doesn't fall out. Many women find enemas irritating to the rectal tissues and the risk of infection is no different without them. Next the woman is put on a table lying down on her back.
Most hospitals use metal stirrups.
This position is only used in the U.S. routinely. And it doesn't use gravity. It is so the doctor can see what is happening. Her legs are in the air, her buttocks at the table's edge, her vagina totally exposed. As the ultimate symbolic inversion, it is ritually appropriate that this position be reserved for the peak transformational moments of the initiation experience - the birth itself.