For more than a generation, Americans have aggressively sought to guarantee girls every opportunity in the world. Except one: the chance to be born.

Forty years ago this month, the passage of Title IX legislation kicked off what scholar Barbara Dafoe Whitehead has called "The Girl Project" — a multi-decade cultural overhaul aimed at assuring girls' access to everything from Little League to the Citadel military academy.

The following year, in 1973, the Supreme Court's Roe v. Wade decision legalized abortion on demand across the nation.

Fast-forward to the end of May, when the U.S. House of Representatives considered a bill to ban sex-selective abortion.

Sex-selective abortion abroad has gotten significant attention since this spring's dramatic escape of blind Chinese activist Chen Guangcheng. Chen had exposed the horrors of China's one-child policy, which has led to an estimated 40 million missing females as a result of sex-selective abortion and infanticide.

But female gendercide — as the Economist magazine and others have called it — is not just a phenomenon in China. Around the world, as many as 200 million unborn girls have been eliminated by the practice. It's even happening here in the United States.

Although sex-selective abortion isn't the rampant trend it is elsewhere, skewed birth ratios among certain segments of the U.S. population (with boys outnumbering girls at an unusual rate) can be explained only by unnatural causes. The same week that Chen finally arrived in America, the House debated the Prenatal Non-Discrimination Act (PRENDA) to bar sex-selective abortion in this country.

Remarkably, the Obama administration opposed PRENDA. So did some groups that purportedly stand for women, including the National Organization for Women (NOW), the National Women's Law Center, Planned Parenthood and NARAL Pro-Choice America.

Some tried — preposterously — to paint the bill as another onslaught in the "war on women" — a political fabrication of the left to avoid engaging in substantive argument about a variety of issues. They argued that policymakers shouldn't interfere in a woman's private right to choose.

That's ironic, since PRENDA proponents are defending the right of females to ever have any choice in the first place.

"Aborting a baby based upon their gender undermines one of our nation's founding principles that all human beings are created equal," said Rep. Diane Black, R-Tenn., an original co-sponsor of PRENDA. "United States law currently prohibits discrimination on the basis of gender. Abortion should be no exception."

Regrettably, pro-abortion feminist groups convinced enough House members to block PRENDA on May 31. But they are out of step with most Americans. A Gallup poll in May found that the percentage of Americans who consider themselves "pro-choice" has fallen to 41 percent — the lowest level since the question has been asked.

A recent poll by the Charlotte Lozier Institute found that 77 percent of Americans support a ban on sex-selective abortion. Four states have outlawed this anti-woman practice: Illinois, Oklahoma, Pennsylvania and Arizona.

Still, the practice continues. A group called LiveAction released three videos from undercover visits to abortion clinics in which workers aided plans by a "client" to terminate a pregnancy because the unborn child was a girl. The first two videos exposed the practice at clinics in New York and Texas. The third, released June 6, was shot at a clinic in Arizona, where sex-selective abortion is illegal. (The videos are online at Protect-OurGirls.com.)

Opponents of the House bill banning sex-selective abortion argued that instead of changing the law to protect unborn girls from extermination simply because they are girls, we should work to promote a culture that will value females more.

That's certainly a worthy goal as well. But what better signal to communicate that girls deserve respect than the unequivocal affirmation that girls deserve to be born?

Jennifer A. Marshall is director of the DeVos Center for Religion and Civil Society at The Heritage Foundation in Washington, D.C.