COLUMBUS, Ohio — An anti-abortion group in Ohio fell short today in its attempt to gather enough signatures to change the state constitution to declare that life begins when a human egg is fertilized.

Backers of the proposed constitutional amendment in Ohio and elsewhere hope to spark a legal challenge to the U.S. Supreme Court’s 1973 Roe v. Wade decision that gave women a legal right to abortion.

The group had only collected about 30,000 of the roughly 385,000 signatures required for November ballots, Patrick Johnston, the director of Personhood Ohio, said today.

The group pledged to continue seeking signatures ahead of 2013, but the shortfall was another obstacle for what has become known as the “personhood” movement.

Supporters fell short of the required number of signatures to qualify for the November ballots in Nevada and California. And in Oklahoma, the state’s highest court halted an amendment effort there to grant personhood rights to human embryos, saying the measure was unconstitutional.

Voters have rejected similar proposals that made ballots in 2008 and 2010 in Colorado. They also defeated the initiative last November in Mississippi, which has some of the nation’s toughest abortion regulations.

Organizers say personhood amendments have a good chance to qualify for fall ballots in Montana and again in Colorado. Each state has a lower threshold of required signatures than Ohio. About 86,000 signatures are needed by early August in Colorado, while fewer than 49,000 are required by Friday in Montana.

The measures vary in some details, but in general they define human life as beginning with fertilization and are intended to ban virtually all abortions, even in cases of rape and incest. Many physicians have said the measures could make some birth control illegal and deter in vitro fertilization.

Supporters in Ohio had hoped to alleviate those concerns by rephrasing their proposed amendment to say it wouldn’t affect “genuine contraception” or in vitro fertilization procedures.

The proposal hasn’t attracted support from some traditional supporters of Ohio’s anti-abortion measures.

The anti-abortion group Ohio Right to Life is not involved. And while it hasn’t taken a formal position, the Catholic Conference of Ohio is not encouraging its 785 parishes to gather signatures.

The amendment is also trying to gain traction at the same time as the so-called heartbeat bill, which would outlaw abortions at the first detectable fetal heartbeat, sometimes as early as six weeks into pregnancy.

Activists from across the political spectrum say getting an issue on Ohio’s ballots is a daunting task.

The We Are Ohio campaign, which successfully sought the 2011 repeal of the state’s collective bargaining law, hired paid signature gathers even with the support of well-funded labor unions, the Ohio Democratic Party and thousands of volunteers.