"When do you believe life begins?" Johnny DuPree, Democratic candidate for governor of Mississippi, asked during a public debate on October 14. The question was rhetorical, and DuPree's answer—not a surprise in one of the most socially conservative US states—was the same as that of his Republican opponent: "I believe life begins at conception."

On November 8, Mississippi voters will not only decide who should lead the state, but also indicate whether they agree with the candidates about the status of embryos. The Initiative 26 ballot measure proposes to amend the state's constitution to redefine 'person' as "every human being from the moment of fertilization, cloning, or the equivalent thereof". If approved, the amendment would effectively bestow human rights on fertilized human eggs, making abortion illegal in the state in most, if not all, circumstances.

"The unborn child in the womb is scientifically proven to be a human being, and when it comes down to it we are a human-rights organization," says Jennifer Mason, communications director for Personhood USA, a national advocacy group based in Arvada, Colorado, and one of the initiative's backers.

By defining personhood so broadly, the measure would also have an impact beyond abortion—for example, it could rule out research using human embryonic stem cells and put doctors who offer in vitro fertilization (IVF) in a dubious legal position, because not all embryos created during fertility treatment survive the procedure.

"This is a dangerous and extreme government intrusion into women's health, women's rights and families' health," says Stan Flint, a consultant to Mississippians for Healthy Families, based in Jackson, which opposes the amendment.

Similar propositions have been put to voters in the United States twice before—during statewide campaigns in Colorado, where the personhood movement first emerged as a strategic challenge to abortion laws. But in both 2008 and 2010, personhood initiatives were roundly defeated, respectively winning only 27% and 29% of votes cast.

Mississippi could be very different. With candidates from both parties, the state's attorney general and a phalanx of religious groups all endorsing Initiative 26, there is a widespread sense that the measure could pass. Such a victory would undoubtedly boost efforts to pass personhood initiatives in several other states, from Oregon to Florida (see 'States of play'), where similar measures are set to appear on ballots in November 2012. Ultimately, Person­hood USA hopes to win initiatives in at least two-thirds of the 50 states. Those states, in turn, could then require Congress to put forward a personhood amendment to the US Constitution.

Even if that strategy falls short, victory in just a few states would shift the balance in the ongoing US debate over the handling of human embryos, inviting future restrictions or conditions on federal funding for stem-cell research. "I think the field should be worried," says Alta Charo, an expert on law and bio­ethics at the University of Wisconsin–Madison. "They are still being held hostage by the abortion debates."

The Mississippi vote itself will have little direct impact on human embryonic stem-cell research, because the state is not a major player in the field. The potential threat to reproductive technology is more immediate.

The personhood movement says claims that the initiative would restrict fertility treatments and birth control are fear mongering. "It's not going to ban IVF and contraception. It's only going to ban procedures and pills that kill a person," says Mason. "If it kills a person it should be illegal."

But opponents say that the wording of the amendment means that it could interfere with established medical practices. "If personhood begins at fertilization, then we have to talk about IVF and birth control," says Jonathan Will, director of the Bioethics and Health Law Center at Mississippi College in Jackson. Some forms of contraception, such as the intra­uterine device and emergency hormonal contraceptive pills, prevent fertilized embryos from implanting in the uterine wall and so could be considered illegal under the amendment, experts say.

Fertility doctors add that the measure could hamper IVF and endanger the would-be mother and her offspring.

To give patients the best chance of pregnancy, doctors typically fertilize 8–10 eggs and implant only the one or two embryos that seem most vigorous. The rest are stored or discarded. If a doctor is forced to implant all fertilized eggs to avoid prosecution, then the patient is more likely to have multiple pregnancies, which can be risky for her and the fetuses. Yet limiting the number of embryos created for IVF to only the number of children desired reduces the chance of success and increases the likelihood that women will have to undergo the difficult and expensive procedure more than once.

Proponents of Initiative 26 point out that other countries have already legally limited the number of fertilizations in IVF. In Italy, for example, a law introduced in 2004 limits doctors to fertilizing only three eggs and requires all resulting embryos to be implanted. Yet studies suggest that the law has reduced the success rate of IVF and increased the number of triplet pregnancies (P. E. Levi Setti et al. Fertil. Steril. 90, 1081–1086; 2008).

As the campaign for Initiative 26 heads into its final days, voters have been bombarded with commentaries, blogs, YouTube videos and public rallies on both sides of the debate. Flint acknowledges that, at this point, the defeat of the Mississippi initiative would be a turnaround, but an increasingly vocal opposition movement has thrown predictions of an easy victory for the initiative into question. "Starting from a dead stop at two months out, we have put together a major campaign," says Flint. "The momentum has swung strongly towards the opposition to this amendment."

This article is reproduced with permission from the magazine Nature. The article was first published on November 1, 2011.