Viability and Sentience

Back near the anniversary of Roe v. Wade, a number of liberal bloggers posted commentary questioning the widespread view -- accepted even by many supporters of abortion rights -- that abortion is, at a minimum, a morally problematic practice.  Over at Unfogged, in the course of recounting the story of her own abortion, LizardBreath made the following claim:

Morally, I think that a ten week embryo -- in fact, any fetus in at least the first two trimesters -- is not sentient and is not a person or anything else with rights, and that ending a pregnancy does not have moral significance with respect to the rights or interests of the fetus.

I meant to post something at the time questioning the factual claim that a fetus cannot be sentient during the second trimester.  But this USA Today story goes one better, and provides evidence of viability several weeks back into the second trimester: 

A premature baby that doctors say spent less time in the womb than any other surviving infant is to be released from a Florida hospital Tuesday.

Amillia Sonja Taylor was just 9½ inches long and weighed less than 10 ounces when she was born Oct. 24. She was delivered 21 weeks and six days after conception. Full-term births come after 37 to 40 weeks. "We weren't too optimistic," Dr. William Smalling said Monday. "But she proved us all wrong." Neonatologists who cared for Amillia say she is the first baby known to survive after a gestation period of fewer than 23 weeks. A database run by the University of Iowa's Department of Pediatrics lists seven babies born at 23 weeks between 1994 and 2003.

Amillia has experienced respiratory problems, a very mild brain hemorrhage and some digestive problems, but none of the health concerns are expected to pose long-term problems, her doctors said.  "We can deal with lungs and things like that but, of course, the brain is the most important," Dr. Paul Fassbach said Monday. "But her prognosis is excellent."

Viability is not evidence of sentience, but it does lead to an interesting question.  Given the distinct possibility (indeed, I think, likelihood) that some day down the road medical technology might make a fetus viable before it becomes sentient, what is the significance of the concept of viability to those who emphasize sentience in their analysis of the morality of abortion?