a pregnant body lying face down flat on a table with a belly hanging below IS a strain on uterine ligaments, the back, everything. A pregnant belly hanging down when someone is on all fours is a good back stretch, helps with positioning of the baby, takes the weight of the baby off the bladder/stomach/diaphram/back/pelvis, been done forever as women have worked in fields, scrubbed floor etc., plus the more recent activities of prenatal yoga and stretching classes. It's a recommended position for positioning the baby in a more favourable position than baby's spine against mother's spine.
http://www.spinningbabies.com has a lot of info. (the site does include a picture on a prenatal table for a chiropractic adjustment, but that shouldn't take a long time....)
Positioning on the bodyCushion is halfway between all fours and flat, and I think it is as safe as all fours. I've been using it for pregnancy massage face down right up to due date for over 7 years now with no problem, other than a couple of VERY large bellies (twins) and one client that found it uncomfortable that was eventually diagnosed with a pelvic joint issue, even with those cases, the bodyCushion is great for using side lying, so it is flexible for face down or sidelying, and face up as they are elevated and you can easily slightly elevate the right side. I used to think it didn't matter what side someone would lay on, left seemed to be recommended due to the easier on the heart recovery position, but this site has made me reconsider that form week 30 or so it's probably a good idea to keep to one side, or at least spend most time on one side so gravity can work..
So I am anti-prenatal tables, regardless of how supportive a sling under the belly maybe, but I have no concerns about massage face down when it is in a more on all fours position than back and legs all in one straight line. A lot of the pregnancy cushions I have seen are just full table cushions with holes cut out putting the client in the same position as a prenatal table, but something that can position the back above the knee height make sense to me, and my happy clients
***** fixed up broken links