Image: A five-day-old embryo, from the Institute for Stem Cell Research; a 12-week-old fetus, from Mitch/Flickr.

John McCain's recent statement on embryonic stem cell research was ambiguous in some ways, but clearly misleading in another: He equated human embryos with fetuses, and used language implying that farming fetuses for their tissues is a realistic possibility.

"I voted to ban the practice of 'fetal farming,' making it a federal crime for researchers to use cells or fetal tissue from an embryo created for research purposes," McCain said in a statement Monday in response to questions posed by ScienceDebate2008, a nonpartisan science advocacy group.

That McCain would oppose experimentation on embryos fits with his political record. Outside of tolerating Roe v. Wade as a means to avoid unsafe abortions during his 2000 run for president, he's been staunchly opposed to abortion, though conservatives worried during earlier stages of McCain's campaign that he would adopt a pro-choice platform.

The vote mentioned in his statement came on the Fetal Farming Act of 2006, signed into law by President Bush. But though the bill was unanimously approved in the House and Senate, its sponsors were criticized for failing to make clear that "fetal farming" doesn't exist.

Embryos used to produce embryonic stem cells are harvested after five days, when the embryo is still an undifferentiated blob of about 70 cells. While there is no sharp line for when an embryo becomes a fetus, nine weeks is a good rule of thumb; the industry standard for halting development on research embryos is two weeks. No reputable scientist has supported fetus experimentation. For McCain to revive the language of "fetal farming," say bioethicists, was misleading.

"It brings to mind some sort of science fiction, human ectogenesis factory," said Jesse Reynolds, biotechnology accountability project director at the Center for Genetics and Society. He called the phrase "a misleading rhetorical element designed to fire up the [conservative] base."

The United States has no federal law for halting embryo experimentation, but 14 days -- the point at which an embryo evidences its first distinguishing feature, a crease down its center known as the "primitive streak" -- is accepted in the research community. California and Massachusetts both have 14-day limits by law, as does the United Kingdom, and the National Academies of Science recommends it. Because stem cell lines are shared so widely in the research community, these standards have become a de facto universal law.

"Boasting of opposition to 'fetal farming' is offensive, because it suggests that 'fetal farming' was ever an activity in which scientists were engaged," said bioethicist R. Alta Charo of the University of Wisconsin. "They were not and are not 'farming' fetuses, and this legislation was not merely unnecessary, it was deliberately misleading and inflammatory."

Whether the statement was intended to satisfy pro-life members of the Republican party is speculative, and the McCain campaign did not respond to queries for this article. But fetal farming is a possibility only if fetuses are equated with embryos -- something that McCain did in his description of "fetal tissue from an embryo created for research purposes."

In scientific terms, embryos and fetuses are different and mutually exclusive entities. Equating them fits within a pro-life framework defining abortion at any stage as murder.

"If you call the creation of an embryo for research 'fetal farming,' that clearly conflates embryos, at whatever stage, with fetuses," said Thomas Murray, director of the Hastings Center, a nonpartisan bioethics think tank. "It would seem to equate a five-day-old embryo with a fetus one day before delivery."