Surrogacy Post Fertilization The wikipedia page on surrogacy highlights two main forms of surrogacy: Traditional Surrogacy and Gestational Surrogacy. Traditional surrogacy inseminates the surrogate mother with sperm, while gestational surrogacy uses in vitro fertilization to form an embryo from egg and sperm, which is then transferred into the surrogate mother. It does not discuss whether surrogacy is possible after a natural fertilization has already taken place, such as if a woman becomes pregnant and then wishes, for whatever reason, to have a surrogate take the baby to term. Is this biologically possible? If it is possible, what is the mechanism? Also, if possible, is heightened risk a major reason it is not more common?

No, it is not possible. The problem is the fertilization which takes place near the ovary. It takes the new forming blastocyst about 4-5 days to wander through the fallopian tube to reach the uterus and 8-9 days in total to implant itself into the uterine wall. See the picture below (from the Wikipedia article on fertilization):

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You would have to get the blastocyst out of the right fallopian tube, make sure it survives the procedure and then implant it into the uterus of a woman which is about in the same phase of their period (otherwise the uterus is not ready to allow implantation). It is already hard enough to do in vitro fertilization where you can control the environment.