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It's the mother of all abortion battles: The "personhood" movement, which aims to define embryos as people with legal rights, is gaining ground across the country. Opponents say women's reproductive rights are at stake, with personhood moving well beyond abortion into areas such as infertility treatments and birth control.

In Kansas, the governor recently signed a sweeping bill of abortion restrictions that includes personhood language. In North Dakota, lawmakers agreed to let voters decide in 2014 whether the state constitution should assert that life begins at fertilization, as personhood advocates believe. At press time, activists in Mississippi were collecting signatures to put a personhood amendment on the state ballot; this year, a total of 13 states have introduced bills or ballot initiatives that would legally declare embryos people.

Abortion rights advocates are alarmed. Giving "full constitutional rights to a fertilized egg" would "invite the government into every aspect of reproduction," says Eric Ferrero, vice president of communications at Planned Parenthood Federation of America. Not only would virtually all abortions be banned, personhood critics say, but infertility treatments resulting in the destruction of embryos (such as IVF) could be, too, as could birth control methods that stop a fertilized egg from implanting in the womb. (The FDA says the morning-after pill and the copper IUD can prevent implantation; some scientists hotly contest that the morning-after pill does so.)

Keith Mason, president of Personhood USA, a nonprofit that organizes activists, dismisses critics' claims as "scare tactics." Most birth control would not be affected, he says, and infertility treatments wouldn't be banned but "modified to be respectful of life and to ensure that embryos aren't discarded." The bottom line, he says, is that life begins when egg meets sperm, the moment "your hair, eye color, and height are determined."

President Barack Obama recently jumped into the fray as well. Speaking at a Planned Parenthood conference in late April, he referred to personhood initiatives as "absurd" and "an assault on women's rights." Mason says he wants to thank the president for that. "Obviously we're touching a nerve. He understands that we're a threat."

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