If she delivers the rare Asian gaur growing inside her, she will herald a stunning new way to save endangered, or even recently extinct, animals.
The bovine surrogate mother is carrying the gaur fetus on a farm near Sioux City, Iowa, and is expected to give birth to "Noah" next month.
"He will be the first endangered animal we send up the ramp of the ark," said Robert Lanza, the vice president of medical and scientific development at Advanced Cell Technology, and one of the lead authors of a study published Sunday in the journal Cloning.
"This is no longer science fiction. It's very, very real."
Scientists had previously shown that it is possible for one species to give birth to implanted embryos taken from a similar species. But this is the first time they have combined that technique with cloning.
Using a technique developed by ACT in Worcester, Mass., scientists removed the DNA from one of Bessie's eggs and fused the egg with a skin cell taken from a living gaur, producing a genetically gaur egg that would be accepted by Bessie's immune system.
Noah doesn't have a father. Before being implanted in Bessie's uterus, the egg was artificially induced to begin dividing without being fertilized.
The technique could not be used on long-extinct species because the DNA donor cells must come from a live animal, one that has been dead for less than five days or one that has been frozen since its death.
The goal of the research is to use the cross-species cell transfer technology to reprogram human cells for medical purposes, said Michael West, chief executive officer of ACT.
Until then, if Noah's birth is successful, developers say the technique could be used to repopulate rare animal species.
The Spanish government has given the Massachusetts company approval to clone the bucardo, a newly extinct Spanish mountain goat. The last bucardo died earlier this year and was immediately frozen. Researchers also are considering rare animals such as the African bongo, the ocelot and the giant panda.
"We're not trying to build a big Noah's ark and save every animal, but we want to point the way for how this can be done," he said.
"Saving the world is a tall order, but we're trying."
However, animal preservationists fear the public will prefer the less-costly cloning techniques and pull funding for expensive breeding and habitat protection programs already in place.
"The danger is that this could be seen as an alternative," said John Rennie, editor of Scientific American magazine. "But cloning is just one more tool to use along with the rest of the measures we already take to preserve species."
Even some critics of cloning say the ACT researchers may have stumbled upon a positive use of the technology.
"There are no moral problems with this," said Michael Grodin, a professor in Boston University's School of Public Health who has opposed advances that could lead to the cloning of humans.
"There are a host of reasons why cloning humans is wrong, but this could be a positive step toward maintaining these species."
Bessie was one of 32 cows implanted with the fused eggs, and the only one to bring an embryo to near-term.
Three others came close, but researchers removed the embryos midway through the pregnancy to make sure the fetuses were developing.
"The tests showed that the chromosomes were developing as beautiful little gaurs," Mr. Lanza said.
"This is the very first time this technology has ever generated a full pregnancy."
Although the technique could be used with cells from animals that were frozen immediately after death, it could not resurrect a woolly mammoth from specimens frozen for centuries in Siberian permafrost because their DNA has become fragmented, Mr. Lanza said.
"It's like trying to rebuild the Roman Forum," Mr. Lanza said. "All the pieces are somewhere in Rome, but you have to be able to reassemble them. And that would take a long time."