I would think that because of the tense feeling during anal sex that it would cause stress on the uterus. I know I've only tried it once and my whole body was tense (sp?).
Hmm...good question. I was listening to this sex talk show called Dr. Sue Johansson, or something like that and heard that anal sex is not good during pregnancy because as the baby gets bigger the uterus sits more towards the rectum or something. But I looked it up myself and here's what I found.
This is off http://parenting.ivillage.com/pregnancy/psex/0,,midwife_3pjq,00.html
It says:
Anal sex causes no direct harm to the baby.
When having anal sex it's important to go slowly. The anal sphincter is a ring of muscle that can be voluntarily contracted or relaxed. Penetration before your partner is ready can result in tearing of the sensitive skin.
As part of your foreplay gently insert a lubricated finger, and then two, to be sure your partner is relaxed. Penetration should always be comfortable and enjoyable.
If you decide to continue with other love play following anal sex, it's important to always wash the penis well with warm soap and water before reinsertion to avoid a vaginal infection or oral transmission of E. Coli.
Because anal sex has been shown to increase the risk of sexually transmitted diseases and HIV, it's important to always use a condom along with a water-based lubricant, such as AstroGlide. (You might want to avoid condoms with spermicides, as they can irritate the sensitive rectal tissues.)
I'm not quite sure how it might affect the uterus, I can't imagine it would be much different than vaginal but my concern would be in the constipation department. It might make constipation or hemroids worse down the road. I'm not quite sure though? Have you done any searches on it?