Here are some images from the enema section on Curezone that may prove helpful.
Both the knee chest and Sims position are often used for enemas during pregnancy and each helps to minimize uncomfortable pressure from the baby, as well as allowing the water from the enema to enter higher into the colon. They are best used when you have someone to assist and hold the bag and keep the nozzle in place during the enema.
You can usually find an enema bag such as this one at most pharmacies for about $15 or less.
To minimize discomfort during the procedure, go slow and stop the flow for a minute or so if you begin to cramp, and resume when it passes. Don't hang the bag too high. 1 1/2 to 2 feet is ideal, and any higher results in too much pressure on the colon. Finally, make sure the water is a few degrees above body temperature. 103 degrees F at preparation time is ideal.
Please get back to me if I can be of any assistance.
Wishing you a happy and healthy pregnancy for both you and your baby.
Valerie